From 898bd37a92063e46bc8d7b870781cecd66234f92 Mon Sep 17 00:00:00 2001 From: Mauro Carvalho Chehab Date: Thu, 18 Apr 2019 19:45:00 -0300 Subject: docs: block: convert to ReST Rename the block documentation files to ReST, add an index for them and adjust in order to produce a nice html output via the Sphinx build system. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by: Mauro Carvalho Chehab --- Documentation/block/bfq-iosched.rst | 597 ++++++++++++ Documentation/block/bfq-iosched.txt | 583 ----------- Documentation/block/biodoc.rst | 1168 +++++++++++++++++++++++ Documentation/block/biodoc.txt | 1076 --------------------- Documentation/block/biovecs.rst | 146 +++ Documentation/block/biovecs.txt | 144 --- Documentation/block/capability.rst | 18 + Documentation/block/capability.txt | 15 - Documentation/block/cmdline-partition.rst | 53 + Documentation/block/cmdline-partition.txt | 46 - Documentation/block/data-integrity.rst | 291 ++++++ Documentation/block/data-integrity.txt | 281 ------ Documentation/block/deadline-iosched.rst | 72 ++ Documentation/block/deadline-iosched.txt | 75 -- Documentation/block/index.rst | 25 + Documentation/block/ioprio.rst | 182 ++++ Documentation/block/ioprio.txt | 183 ---- Documentation/block/kyber-iosched.rst | 15 + Documentation/block/kyber-iosched.txt | 14 - Documentation/block/null_blk.rst | 126 +++ Documentation/block/null_blk.txt | 99 -- Documentation/block/pr.rst | 119 +++ Documentation/block/pr.txt | 119 --- Documentation/block/queue-sysfs.rst | 254 +++++ Documentation/block/queue-sysfs.txt | 253 ----- Documentation/block/request.rst | 99 ++ Documentation/block/request.txt | 88 -- Documentation/block/stat.rst | 93 ++ Documentation/block/stat.txt | 86 -- Documentation/block/switching-sched.rst | 39 + Documentation/block/switching-sched.txt | 35 - Documentation/block/writeback_cache_control.rst | 86 ++ Documentation/block/writeback_cache_control.txt | 86 -- 33 files changed, 3383 insertions(+), 3183 deletions(-) create mode 100644 Documentation/block/bfq-iosched.rst delete mode 100644 Documentation/block/bfq-iosched.txt create mode 100644 Documentation/block/biodoc.rst delete mode 100644 Documentation/block/biodoc.txt create mode 100644 Documentation/block/biovecs.rst delete mode 100644 Documentation/block/biovecs.txt create mode 100644 Documentation/block/capability.rst delete mode 100644 Documentation/block/capability.txt create mode 100644 Documentation/block/cmdline-partition.rst delete mode 100644 Documentation/block/cmdline-partition.txt create mode 100644 Documentation/block/data-integrity.rst delete mode 100644 Documentation/block/data-integrity.txt create mode 100644 Documentation/block/deadline-iosched.rst delete mode 100644 Documentation/block/deadline-iosched.txt create mode 100644 Documentation/block/index.rst create mode 100644 Documentation/block/ioprio.rst delete mode 100644 Documentation/block/ioprio.txt create mode 100644 Documentation/block/kyber-iosched.rst delete mode 100644 Documentation/block/kyber-iosched.txt create mode 100644 Documentation/block/null_blk.rst delete mode 100644 Documentation/block/null_blk.txt create mode 100644 Documentation/block/pr.rst delete mode 100644 Documentation/block/pr.txt create mode 100644 Documentation/block/queue-sysfs.rst delete mode 100644 Documentation/block/queue-sysfs.txt create mode 100644 Documentation/block/request.rst delete mode 100644 Documentation/block/request.txt create mode 100644 Documentation/block/stat.rst delete mode 100644 Documentation/block/stat.txt create mode 100644 Documentation/block/switching-sched.rst delete mode 100644 Documentation/block/switching-sched.txt create mode 100644 Documentation/block/writeback_cache_control.rst delete mode 100644 Documentation/block/writeback_cache_control.txt (limited to 'Documentation/block') diff --git a/Documentation/block/bfq-iosched.rst b/Documentation/block/bfq-iosched.rst new file mode 100644 index 000000000000..2c13b2fc1888 --- /dev/null +++ b/Documentation/block/bfq-iosched.rst @@ -0,0 +1,597 @@ +========================== +BFQ (Budget Fair Queueing) +========================== + +BFQ is a proportional-share I/O scheduler, with some extra +low-latency capabilities. In addition to cgroups support (blkio or io +controllers), BFQ's main features are: + +- BFQ guarantees a high system and application responsiveness, and a + low latency for time-sensitive applications, such as audio or video + players; +- BFQ distributes bandwidth, and not just time, among processes or + groups (switching back to time distribution when needed to keep + throughput high). + +In its default configuration, BFQ privileges latency over +throughput. So, when needed for achieving a lower latency, BFQ builds +schedules that may lead to a lower throughput. If your main or only +goal, for a given device, is to achieve the maximum-possible +throughput at all times, then do switch off all low-latency heuristics +for that device, by setting low_latency to 0. See Section 3 for +details on how to configure BFQ for the desired tradeoff between +latency and throughput, or on how to maximize throughput. + +As every I/O scheduler, BFQ adds some overhead to per-I/O-request +processing. To give an idea of this overhead, the total, +single-lock-protected, per-request processing time of BFQ---i.e., the +sum of the execution times of the request insertion, dispatch and +completion hooks---is, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz +(dated CPU for notebooks; time measured with simple code +instrumentation, and using the throughput-sync.sh script of the S +suite [1], in performance-profiling mode). To put this result into +context, the total, single-lock-protected, per-request execution time +of the lightest I/O scheduler available in blk-mq, mq-deadline, is 0.7 +us (mq-deadline is ~800 LOC, against ~10500 LOC for BFQ). + +Scheduling overhead further limits the maximum IOPS that a CPU can +process (already limited by the execution of the rest of the I/O +stack). To give an idea of the limits with BFQ, on slow or average +CPUs, here are, first, the limits of BFQ for three different CPUs, on, +respectively, an average laptop, an old desktop, and a cheap embedded +system, in case full hierarchical support is enabled (i.e., +CONFIG_BFQ_GROUP_IOSCHED is set), but CONFIG_BFQ_CGROUP_DEBUG is not +set (Section 4-2): +- Intel i7-4850HQ: 400 KIOPS +- AMD A8-3850: 250 KIOPS +- ARM CortexTM-A53 Octa-core: 80 KIOPS + +If CONFIG_BFQ_CGROUP_DEBUG is set (and of course full hierarchical +support is enabled), then the sustainable throughput with BFQ +decreases, because all blkio.bfq* statistics are created and updated +(Section 4-2). For BFQ, this leads to the following maximum +sustainable throughputs, on the same systems as above: +- Intel i7-4850HQ: 310 KIOPS +- AMD A8-3850: 200 KIOPS +- ARM CortexTM-A53 Octa-core: 56 KIOPS + +BFQ works for multi-queue devices too. + +.. The table of contents follow. Impatients can just jump to Section 3. + +.. CONTENTS + + 1. When may BFQ be useful? + 1-1 Personal systems + 1-2 Server systems + 2. How does BFQ work? + 3. What are BFQ's tunables and how to properly configure BFQ? + 4. BFQ group scheduling + 4-1 Service guarantees provided + 4-2 Interface + +1. When may BFQ be useful? +========================== + +BFQ provides the following benefits on personal and server systems. + +1-1 Personal systems +-------------------- + +Low latency for interactive applications +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Regardless of the actual background workload, BFQ guarantees that, for +interactive tasks, the storage device is virtually as responsive as if +it was idle. For example, even if one or more of the following +background workloads are being executed: + +- one or more large files are being read, written or copied, +- a tree of source files is being compiled, +- one or more virtual machines are performing I/O, +- a software update is in progress, +- indexing daemons are scanning filesystems and updating their + databases, + +starting an application or loading a file from within an application +takes about the same time as if the storage device was idle. As a +comparison, with CFQ, NOOP or DEADLINE, and in the same conditions, +applications experience high latencies, or even become unresponsive +until the background workload terminates (also on SSDs). + +Low latency for soft real-time applications +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Also soft real-time applications, such as audio and video +players/streamers, enjoy a low latency and a low drop rate, regardless +of the background I/O workload. As a consequence, these applications +do not suffer from almost any glitch due to the background workload. + +Higher speed for code-development tasks +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +If some additional workload happens to be executed in parallel, then +BFQ executes the I/O-related components of typical code-development +tasks (compilation, checkout, merge, ...) much more quickly than CFQ, +NOOP or DEADLINE. + +High throughput +^^^^^^^^^^^^^^^ + +On hard disks, BFQ achieves up to 30% higher throughput than CFQ, and +up to 150% higher throughput than DEADLINE and NOOP, with all the +sequential workloads considered in our tests. With random workloads, +and with all the workloads on flash-based devices, BFQ achieves, +instead, about the same throughput as the other schedulers. + +Strong fairness, bandwidth and delay guarantees +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +BFQ distributes the device throughput, and not just the device time, +among I/O-bound applications in proportion their weights, with any +workload and regardless of the device parameters. From these bandwidth +guarantees, it is possible to compute tight per-I/O-request delay +guarantees by a simple formula. If not configured for strict service +guarantees, BFQ switches to time-based resource sharing (only) for +applications that would otherwise cause a throughput loss. + +1-2 Server systems +------------------ + +Most benefits for server systems follow from the same service +properties as above. In particular, regardless of whether additional, +possibly heavy workloads are being served, BFQ guarantees: + +* audio and video-streaming with zero or very low jitter and drop + rate; + +* fast retrieval of WEB pages and embedded objects; + +* real-time recording of data in live-dumping applications (e.g., + packet logging); + +* responsiveness in local and remote access to a server. + + +2. How does BFQ work? +===================== + +BFQ is a proportional-share I/O scheduler, whose general structure, +plus a lot of code, are borrowed from CFQ. + +- Each process doing I/O on a device is associated with a weight and a + `(bfq_)queue`. + +- BFQ grants exclusive access to the device, for a while, to one queue + (process) at a time, and implements this service model by + associating every queue with a budget, measured in number of + sectors. + + - After a queue is granted access to the device, the budget of the + queue is decremented, on each request dispatch, by the size of the + request. + + - The in-service queue is expired, i.e., its service is suspended, + only if one of the following events occurs: 1) the queue finishes + its budget, 2) the queue empties, 3) a "budget timeout" fires. + + - The budget timeout prevents processes doing random I/O from + holding the device for too long and dramatically reducing + throughput. + + - Actually, as in CFQ, a queue associated with a process issuing + sync requests may not be expired immediately when it empties. In + contrast, BFQ may idle the device for a short time interval, + giving the process the chance to go on being served if it issues + a new request in time. Device idling typically boosts the + throughput on rotational devices and on non-queueing flash-based + devices, if processes do synchronous and sequential I/O. In + addition, under BFQ, device idling is also instrumental in + guaranteeing the desired throughput fraction to processes + issuing sync requests (see the description of the slice_idle + tunable in this document, or [1, 2], for more details). + + - With respect to idling for service guarantees, if several + processes are competing for the device at the same time, but + all processes and groups have the same weight, then BFQ + guarantees the expected throughput distribution without ever + idling the device. Throughput is thus as high as possible in + this common scenario. + + - On flash-based storage with internal queueing of commands + (typically NCQ), device idling happens to be always detrimental + for throughput. So, with these devices, BFQ performs idling + only when strictly needed for service guarantees, i.e., for + guaranteeing low latency or fairness. In these cases, overall + throughput may be sub-optimal. No solution currently exists to + provide both strong service guarantees and optimal throughput + on devices with internal queueing. + + - If low-latency mode is enabled (default configuration), BFQ + executes some special heuristics to detect interactive and soft + real-time applications (e.g., video or audio players/streamers), + and to reduce their latency. The most important action taken to + achieve this goal is to give to the queues associated with these + applications more than their fair share of the device + throughput. For brevity, we call just "weight-raising" the whole + sets of actions taken by BFQ to privilege these queues. In + particular, BFQ provides a milder form of weight-raising for + interactive applications, and a stronger form for soft real-time + applications. + + - BFQ automatically deactivates idling for queues born in a burst of + queue creations. In fact, these queues are usually associated with + the processes of applications and services that benefit mostly + from a high throughput. Examples are systemd during boot, or git + grep. + + - As CFQ, BFQ merges queues performing interleaved I/O, i.e., + performing random I/O that becomes mostly sequential if + merged. Differently from CFQ, BFQ achieves this goal with a more + reactive mechanism, called Early Queue Merge (EQM). EQM is so + responsive in detecting interleaved I/O (cooperating processes), + that it enables BFQ to achieve a high throughput, by queue + merging, even for queues for which CFQ needs a different + mechanism, preemption, to get a high throughput. As such EQM is a + unified mechanism to achieve a high throughput with interleaved + I/O. + + - Queues are scheduled according to a variant of WF2Q+, named + B-WF2Q+, and implemented using an augmented rb-tree to preserve an + O(log N) overall complexity. See [2] for more details. B-WF2Q+ is + also ready for hierarchical scheduling, details in Section 4. + + - B-WF2Q+ guarantees a tight deviation with respect to an ideal, + perfectly fair, and smooth service. In particular, B-WF2Q+ + guarantees that each queue receives a fraction of the device + throughput proportional to its weight, even if the throughput + fluctuates, and regardless of: the device parameters, the current + workload and the budgets assigned to the queue. + + - The last, budget-independence, property (although probably + counterintuitive in the first place) is definitely beneficial, for + the following reasons: + + - First, with any proportional-share scheduler, the maximum + deviation with respect to an ideal service is proportional to + the maximum budget (slice) assigned to queues. As a consequence, + BFQ can keep this deviation tight not only because of the + accurate service of B-WF2Q+, but also because BFQ *does not* + need to assign a larger budget to a queue to let the queue + receive a higher fraction of the device throughput. + + - Second, BFQ is free to choose, for every process (queue), the + budget that best fits the needs of the process, or best + leverages the I/O pattern of the process. In particular, BFQ + updates queue budgets with a simple feedback-loop algorithm that + allows a high throughput to be achieved, while still providing + tight latency guarantees to time-sensitive applications. When + the in-service queue expires, this algorithm computes the next + budget of the queue so as to: + + - Let large budgets be eventually assigned to the queues + associated with I/O-bound applications performing sequential + I/O: in fact, the longer these applications are served once + got access to the device, the higher the throughput is. + + - Let small budgets be eventually assigned to the queues + associated with time-sensitive applications (which typically + perform sporadic and short I/O), because, the smaller the + budget assigned to a queue waiting for service is, the sooner + B-WF2Q+ will serve that queue (Subsec 3.3 in [2]). + +- If several processes are competing for the device at the same time, + but all processes and groups have the same weight, then BFQ + guarantees the expected throughput distribution without ever idling + the device. It uses preemption instead. Throughput is then much + higher in this common scenario. + +- ioprio classes are served in strict priority order, i.e., + lower-priority queues are not served as long as there are + higher-priority queues. Among queues in the same class, the + bandwidth is distributed in proportion to the weight of each + queue. A very thin extra bandwidth is however guaranteed to + the Idle class, to prevent it from starving. + + +3. What are BFQ's tunables and how to properly configure BFQ? +============================================================= + +Most BFQ tunables affect service guarantees (basically latency and +fairness) and throughput. For full details on how to choose the +desired tradeoff between service guarantees and throughput, see the +parameters slice_idle, strict_guarantees and low_latency. For details +on how to maximise throughput, see slice_idle, timeout_sync and +max_budget. The other performance-related parameters have been +inherited from, and have been preserved mostly for compatibility with +CFQ. So far, no performance improvement has been reported after +changing the latter parameters in BFQ. + +In particular, the tunables back_seek-max, back_seek_penalty, +fifo_expire_async and fifo_expire_sync below are the same as in +CFQ. Their description is just copied from that for CFQ. Some +considerations in the description of slice_idle are copied from CFQ +too. + +per-process ioprio and weight +----------------------------- + +Unless the cgroups interface is used (see "4. BFQ group scheduling"), +weights can be assigned to processes only indirectly, through I/O +priorities, and according to the relation: +weight = (IOPRIO_BE_NR - ioprio) * 10. + +Beware that, if low-latency is set, then BFQ automatically raises the +weight of the queues associated with interactive and soft real-time +applications. Unset this tunable if you need/want to control weights. + +slice_idle +---------- + +This parameter specifies how long BFQ should idle for next I/O +request, when certain sync BFQ queues become empty. By default +slice_idle is a non-zero value. Idling has a double purpose: boosting +throughput and making sure that the desired throughput distribution is +respected (see the description of how BFQ works, and, if needed, the +papers referred there). + +As for throughput, idling can be very helpful on highly seeky media +like single spindle SATA/SAS disks where we can cut down on overall +number of seeks and see improved throughput. + +Setting slice_idle to 0 will remove all the idling on queues and one +should see an overall improved throughput on faster storage devices +like multiple SATA/SAS disks in hardware RAID configuration, as well +as flash-based storage with internal command queueing (and +parallelism). + +So depending on storage and workload, it might be useful to set +slice_idle=0. In general for SATA/SAS disks and software RAID of +SATA/SAS disks keeping slice_idle enabled should be useful. For any +configurations where there are multiple spindles behind single LUN +(Host based hardware RAID controller or for storage arrays), or with +flash-based fast storage, setting slice_idle=0 might end up in better +throughput and acceptable latencies. + +Idling is however necessary to have service guarantees enforced in +case of differentiated weights or differentiated I/O-request lengths. +To see why, suppose that a given BFQ queue A must get several I/O +requests served for each request served for another queue B. Idling +ensures that, if A makes a new I/O request slightly after becoming +empty, then no request of B is dispatched in the middle, and thus A +does not lose the possibility to get more than one request dispatched +before the next request of B is dispatched. Note that idling +guarantees the desired differentiated treatment of queues only in +terms of I/O-request dispatches. To guarantee that the actual service +order then corresponds to the dispatch order, the strict_guarantees +tunable must be set too. + +There is an important flipside for idling: apart from the above cases +where it is beneficial also for throughput, idling can severely impact +throughput. One important case is random workload. Because of this +issue, BFQ tends to avoid idling as much as possible, when it is not +beneficial also for throughput (as detailed in Section 2). As a +consequence of this behavior, and of further issues described for the +strict_guarantees tunable, short-term service guarantees may be +occasionally violated. And, in some cases, these guarantees may be +more important than guaranteeing maximum throughput. For example, in +video playing/streaming, a very low drop rate may be more important +than maximum throughput. In these cases, consider setting the +strict_guarantees parameter. + +slice_idle_us +------------- + +Controls the same tuning parameter as slice_idle, but in microseconds. +Either tunable can be used to set idling behavior. Afterwards, the +other tunable will reflect the newly set value in sysfs. + +strict_guarantees +----------------- + +If this parameter is set (default: unset), then BFQ + +- always performs idling when the in-service queue becomes empty; + +- forces the device to serve one I/O request at a time, by dispatching a + new request only if there is no outstanding request. + +In the presence of differentiated weights or I/O-request sizes, both +the above conditions are needed to guarantee that every BFQ queue +receives its allotted share of the bandwidth. The first condition is +needed for the reasons explained in the description of the slice_idle +tunable. The second condition is needed because all modern storage +devices reorder internally-queued requests, which may trivially break +the service guarantees enforced by the I/O scheduler. + +Setting strict_guarantees may evidently affect throughput. + +back_seek_max +------------- + +This specifies, given in Kbytes, the maximum "distance" for backward seeking. +The distance is the amount of space from the current head location to the +sectors that are backward in terms of distance. + +This parameter allows the scheduler to anticipate requests in the "backward" +direction and consider them as being the "next" if they are within this +distance from the current head location. + +back_seek_penalty +----------------- + +This parameter is used to compute the cost of backward seeking. If the +backward distance of request is just 1/back_seek_penalty from a "front" +request, then the seeking cost of two requests is considered equivalent. + +So scheduler will not bias toward one or the other request (otherwise scheduler +will bias toward front request). Default value of back_seek_penalty is 2. + +fifo_expire_async +----------------- + +This parameter is used to set the timeout of asynchronous requests. Default +value of this is 248ms. + +fifo_expire_sync +---------------- + +This parameter is used to set the timeout of synchronous requests. Default +value of this is 124ms. In case to favor synchronous requests over asynchronous +one, this value should be decreased relative to fifo_expire_async. + +low_latency +----------- + +This parameter is used to enable/disable BFQ's low latency mode. By +default, low latency mode is enabled. If enabled, interactive and soft +real-time applications are privileged and experience a lower latency, +as explained in more detail in the description of how BFQ works. + +DISABLE this mode if you need full control on bandwidth +distribution. In fact, if it is enabled, then BFQ automatically +increases the bandwidth share of privileged applications, as the main +means to guarantee a lower latency to them. + +In addition, as already highlighted at the beginning of this document, +DISABLE this mode if your only goal is to achieve a high throughput. +In fact, privileging the I/O of some application over the rest may +entail a lower throughput. To achieve the highest-possible throughput +on a non-rotational device, setting slice_idle to 0 may be needed too +(at the cost of giving up any strong guarantee on fairness and low +latency). + +timeout_sync +------------ + +Maximum amount of device time that can be given to a task (queue) once +it has been selected for service. On devices with costly seeks, +increasing this time usually increases maximum throughput. On the +opposite end, increasing this time coarsens the granularity of the +short-term bandwidth and latency guarantees, especially if the +following parameter is set to zero. + +max_budget +---------- + +Maximum amount of service, measured in sectors, that can be provided +to a BFQ queue once it is set in service (of course within the limits +of the above timeout). According to what said in the description of +the algorithm, larger values increase the throughput in proportion to +the percentage of sequential I/O requests issued. The price of larger +values is that they coarsen the granularity of short-term bandwidth +and latency guarantees. + +The default value is 0, which enables auto-tuning: BFQ sets max_budget +to the maximum number of sectors that can be served during +timeout_sync, according to the estimated peak rate. + +For specific devices, some users have occasionally reported to have +reached a higher throughput by setting max_budget explicitly, i.e., by +setting max_budget to a higher value than 0. In particular, they have +set max_budget to higher values than those to which BFQ would have set +it with auto-tuning. An alternative way to achieve this goal is to +just increase the value of timeout_sync, leaving max_budget equal to 0. + +weights +------- + +Read-only parameter, used to show the weights of the currently active +BFQ queues. + + +4. Group scheduling with BFQ +============================ + +BFQ supports both cgroups-v1 and cgroups-v2 io controllers, namely +blkio and io. In particular, BFQ supports weight-based proportional +share. To activate cgroups support, set BFQ_GROUP_IOSCHED. + +4-1 Service guarantees provided +------------------------------- + +With BFQ, proportional share means true proportional share of the +device bandwidth, according to group weights. For example, a group +with weight 200 gets twice the bandwidth, and not just twice the time, +of a group with weight 100. + +BFQ supports hierarchies (group trees) of any depth. Bandwidth is +distributed among groups and processes in the expected way: for each +group, the children of the group share the whole bandwidth of the +group in proportion to their weights. In particular, this implies +that, for each leaf group, every process of the group receives the +same share of the whole group bandwidth, unless the ioprio of the +process is modified. + +The resource-sharing guarantee for a group may partially or totally +switch from bandwidth to time, if providing bandwidth guarantees to +the group lowers the throughput too much. This switch occurs on a +per-process basis: if a process of a leaf group causes throughput loss +if served in such a way to receive its share of the bandwidth, then +BFQ switches back to just time-based proportional share for that +process. + +4-2 Interface +------------- + +To get proportional sharing of bandwidth with BFQ for a given device, +BFQ must of course be the active scheduler for that device. + +Within each group directory, the names of the files associated with +BFQ-specific cgroup parameters and stats begin with the "bfq." +prefix. So, with cgroups-v1 or cgroups-v2, the full prefix for +BFQ-specific files is "blkio.bfq." or "io.bfq." For example, the group +parameter to set the weight of a group with BFQ is blkio.bfq.weight +or io.bfq.weight. + +As for cgroups-v1 (blkio controller), the exact set of stat files +created, and kept up-to-date by bfq, depends on whether +CONFIG_BFQ_CGROUP_DEBUG is set. If it is set, then bfq creates all +the stat files documented in +Documentation/cgroup-v1/blkio-controller.rst. If, instead, +CONFIG_BFQ_CGROUP_DEBUG is not set, then bfq creates only the files:: + + blkio.bfq.io_service_bytes + blkio.bfq.io_service_bytes_recursive + blkio.bfq.io_serviced + blkio.bfq.io_serviced_recursive + +The value of CONFIG_BFQ_CGROUP_DEBUG greatly influences the maximum +throughput sustainable with bfq, because updating the blkio.bfq.* +stats is rather costly, especially for some of the stats enabled by +CONFIG_BFQ_CGROUP_DEBUG. + +Parameters to set +----------------- + +For each group, there is only the following parameter to set. + +weight (namely blkio.bfq.weight or io.bfq-weight): the weight of the +group inside its parent. Available values: 1..10000 (default 100). The +linear mapping between ioprio and weights, described at the beginning +of the tunable section, is still valid, but all weights higher than +IOPRIO_BE_NR*10 are mapped to ioprio 0. + +Recall that, if low-latency is set, then BFQ automatically raises the +weight of the queues associated with interactive and soft real-time +applications. Unset this tunable if you need/want to control weights. + + +[1] + P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O + Scheduler", Proceedings of the First Workshop on Mobile System + Technologies (MST-2015), May 2015. + + http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf + +[2] + P. Valente and M. Andreolini, "Improving Application + Responsiveness with the BFQ Disk I/O Scheduler", Proceedings of + the 5th Annual International Systems and Storage Conference + (SYSTOR '12), June 2012. + + Slightly extended version: + + http://algogroup.unimore.it/people/paolo/disk_sched/bfq-v1-suite-results.pdf + +[3] + https://github.com/Algodev-github/S diff --git a/Documentation/block/bfq-iosched.txt b/Documentation/block/bfq-iosched.txt deleted file mode 100644 index bbd6eb5bbb07..000000000000 --- a/Documentation/block/bfq-iosched.txt +++ /dev/null @@ -1,583 +0,0 @@ -BFQ (Budget Fair Queueing) -========================== - -BFQ is a proportional-share I/O scheduler, with some extra -low-latency capabilities. In addition to cgroups support (blkio or io -controllers), BFQ's main features are: -- BFQ guarantees a high system and application responsiveness, and a - low latency for time-sensitive applications, such as audio or video - players; -- BFQ distributes bandwidth, and not just time, among processes or - groups (switching back to time distribution when needed to keep - throughput high). - -In its default configuration, BFQ privileges latency over -throughput. So, when needed for achieving a lower latency, BFQ builds -schedules that may lead to a lower throughput. If your main or only -goal, for a given device, is to achieve the maximum-possible -throughput at all times, then do switch off all low-latency heuristics -for that device, by setting low_latency to 0. See Section 3 for -details on how to configure BFQ for the desired tradeoff between -latency and throughput, or on how to maximize throughput. - -As every I/O scheduler, BFQ adds some overhead to per-I/O-request -processing. To give an idea of this overhead, the total, -single-lock-protected, per-request processing time of BFQ---i.e., the -sum of the execution times of the request insertion, dispatch and -completion hooks---is, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz -(dated CPU for notebooks; time measured with simple code -instrumentation, and using the throughput-sync.sh script of the S -suite [1], in performance-profiling mode). To put this result into -context, the total, single-lock-protected, per-request execution time -of the lightest I/O scheduler available in blk-mq, mq-deadline, is 0.7 -us (mq-deadline is ~800 LOC, against ~10500 LOC for BFQ). - -Scheduling overhead further limits the maximum IOPS that a CPU can -process (already limited by the execution of the rest of the I/O -stack). To give an idea of the limits with BFQ, on slow or average -CPUs, here are, first, the limits of BFQ for three different CPUs, on, -respectively, an average laptop, an old desktop, and a cheap embedded -system, in case full hierarchical support is enabled (i.e., -CONFIG_BFQ_GROUP_IOSCHED is set), but CONFIG_BFQ_CGROUP_DEBUG is not -set (Section 4-2): -- Intel i7-4850HQ: 400 KIOPS -- AMD A8-3850: 250 KIOPS -- ARM CortexTM-A53 Octa-core: 80 KIOPS - -If CONFIG_BFQ_CGROUP_DEBUG is set (and of course full hierarchical -support is enabled), then the sustainable throughput with BFQ -decreases, because all blkio.bfq* statistics are created and updated -(Section 4-2). For BFQ, this leads to the following maximum -sustainable throughputs, on the same systems as above: -- Intel i7-4850HQ: 310 KIOPS -- AMD A8-3850: 200 KIOPS -- ARM CortexTM-A53 Octa-core: 56 KIOPS - -BFQ works for multi-queue devices too. - -The table of contents follow. Impatients can just jump to Section 3. - -CONTENTS - -1. When may BFQ be useful? - 1-1 Personal systems - 1-2 Server systems -2. How does BFQ work? -3. What are BFQ's tunables and how to properly configure BFQ? -4. BFQ group scheduling - 4-1 Service guarantees provided - 4-2 Interface - -1. When may BFQ be useful? -========================== - -BFQ provides the following benefits on personal and server systems. - -1-1 Personal systems --------------------- - -Low latency for interactive applications - -Regardless of the actual background workload, BFQ guarantees that, for -interactive tasks, the storage device is virtually as responsive as if -it was idle. For example, even if one or more of the following -background workloads are being executed: -- one or more large files are being read, written or copied, -- a tree of source files is being compiled, -- one or more virtual machines are performing I/O, -- a software update is in progress, -- indexing daemons are scanning filesystems and updating their - databases, -starting an application or loading a file from within an application -takes about the same time as if the storage device was idle. As a -comparison, with CFQ, NOOP or DEADLINE, and in the same conditions, -applications experience high latencies, or even become unresponsive -until the background workload terminates (also on SSDs). - -Low latency for soft real-time applications - -Also soft real-time applications, such as audio and video -players/streamers, enjoy a low latency and a low drop rate, regardless -of the background I/O workload. As a consequence, these applications -do not suffer from almost any glitch due to the background workload. - -Higher speed for code-development tasks - -If some additional workload happens to be executed in parallel, then -BFQ executes the I/O-related components of typical code-development -tasks (compilation, checkout, merge, ...) much more quickly than CFQ, -NOOP or DEADLINE. - -High throughput - -On hard disks, BFQ achieves up to 30% higher throughput than CFQ, and -up to 150% higher throughput than DEADLINE and NOOP, with all the -sequential workloads considered in our tests. With random workloads, -and with all the workloads on flash-based devices, BFQ achieves, -instead, about the same throughput as the other schedulers. - -Strong fairness, bandwidth and delay guarantees - -BFQ distributes the device throughput, and not just the device time, -among I/O-bound applications in proportion their weights, with any -workload and regardless of the device parameters. From these bandwidth -guarantees, it is possible to compute tight per-I/O-request delay -guarantees by a simple formula. If not configured for strict service -guarantees, BFQ switches to time-based resource sharing (only) for -applications that would otherwise cause a throughput loss. - -1-2 Server systems ------------------- - -Most benefits for server systems follow from the same service -properties as above. In particular, regardless of whether additional, -possibly heavy workloads are being served, BFQ guarantees: - -. audio and video-streaming with zero or very low jitter and drop - rate; - -. fast retrieval of WEB pages and embedded objects; - -. real-time recording of data in live-dumping applications (e.g., - packet logging); - -. responsiveness in local and remote access to a server. - - -2. How does BFQ work? -===================== - -BFQ is a proportional-share I/O scheduler, whose general structure, -plus a lot of code, are borrowed from CFQ. - -- Each process doing I/O on a device is associated with a weight and a - (bfq_)queue. - -- BFQ grants exclusive access to the device, for a while, to one queue - (process) at a time, and implements this service model by - associating every queue with a budget, measured in number of - sectors. - - - After a queue is granted access to the device, the budget of the - queue is decremented, on each request dispatch, by the size of the - request. - - - The in-service queue is expired, i.e., its service is suspended, - only if one of the following events occurs: 1) the queue finishes - its budget, 2) the queue empties, 3) a "budget timeout" fires. - - - The budget timeout prevents processes doing random I/O from - holding the device for too long and dramatically reducing - throughput. - - - Actually, as in CFQ, a queue associated with a process issuing - sync requests may not be expired immediately when it empties. In - contrast, BFQ may idle the device for a short time interval, - giving the process the chance to go on being served if it issues - a new request in time. Device idling typically boosts the - throughput on rotational devices and on non-queueing flash-based - devices, if processes do synchronous and sequential I/O. In - addition, under BFQ, device idling is also instrumental in - guaranteeing the desired throughput fraction to processes - issuing sync requests (see the description of the slice_idle - tunable in this document, or [1, 2], for more details). - - - With respect to idling for service guarantees, if several - processes are competing for the device at the same time, but - all processes and groups have the same weight, then BFQ - guarantees the expected throughput distribution without ever - idling the device. Throughput is thus as high as possible in - this common scenario. - - - On flash-based storage with internal queueing of commands - (typically NCQ), device idling happens to be always detrimental - for throughput. So, with these devices, BFQ performs idling - only when strictly needed for service guarantees, i.e., for - guaranteeing low latency or fairness. In these cases, overall - throughput may be sub-optimal. No solution currently exists to - provide both strong service guarantees and optimal throughput - on devices with internal queueing. - - - If low-latency mode is enabled (default configuration), BFQ - executes some special heuristics to detect interactive and soft - real-time applications (e.g., video or audio players/streamers), - and to reduce their latency. The most important action taken to - achieve this goal is to give to the queues associated with these - applications more than their fair share of the device - throughput. For brevity, we call just "weight-raising" the whole - sets of actions taken by BFQ to privilege these queues. In - particular, BFQ provides a milder form of weight-raising for - interactive applications, and a stronger form for soft real-time - applications. - - - BFQ automatically deactivates idling for queues born in a burst of - queue creations. In fact, these queues are usually associated with - the processes of applications and services that benefit mostly - from a high throughput. Examples are systemd during boot, or git - grep. - - - As CFQ, BFQ merges queues performing interleaved I/O, i.e., - performing random I/O that becomes mostly sequential if - merged. Differently from CFQ, BFQ achieves this goal with a more - reactive mechanism, called Early Queue Merge (EQM). EQM is so - responsive in detecting interleaved I/O (cooperating processes), - that it enables BFQ to achieve a high throughput, by queue - merging, even for queues for which CFQ needs a different - mechanism, preemption, to get a high throughput. As such EQM is a - unified mechanism to achieve a high throughput with interleaved - I/O. - - - Queues are scheduled according to a variant of WF2Q+, named - B-WF2Q+, and implemented using an augmented rb-tree to preserve an - O(log N) overall complexity. See [2] for more details. B-WF2Q+ is - also ready for hierarchical scheduling, details in Section 4. - - - B-WF2Q+ guarantees a tight deviation with respect to an ideal, - perfectly fair, and smooth service. In particular, B-WF2Q+ - guarantees that each queue receives a fraction of the device - throughput proportional to its weight, even if the throughput - fluctuates, and regardless of: the device parameters, the current - workload and the budgets assigned to the queue. - - - The last, budget-independence, property (although probably - counterintuitive in the first place) is definitely beneficial, for - the following reasons: - - - First, with any proportional-share scheduler, the maximum - deviation with respect to an ideal service is proportional to - the maximum budget (slice) assigned to queues. As a consequence, - BFQ can keep this deviation tight not only because of the - accurate service of B-WF2Q+, but also because BFQ *does not* - need to assign a larger budget to a queue to let the queue - receive a higher fraction of the device throughput. - - - Second, BFQ is free to choose, for every process (queue), the - budget that best fits the needs of the process, or best - leverages the I/O pattern of the process. In particular, BFQ - updates queue budgets with a simple feedback-loop algorithm that - allows a high throughput to be achieved, while still providing - tight latency guarantees to time-sensitive applications. When - the in-service queue expires, this algorithm computes the next - budget of the queue so as to: - - - Let large budgets be eventually assigned to the queues - associated with I/O-bound applications performing sequential - I/O: in fact, the longer these applications are served once - got access to the device, the higher the throughput is. - - - Let small budgets be eventually assigned to the queues - associated with time-sensitive applications (which typically - perform sporadic and short I/O), because, the smaller the - budget assigned to a queue waiting for service is, the sooner - B-WF2Q+ will serve that queue (Subsec 3.3 in [2]). - -- If several processes are competing for the device at the same time, - but all processes and groups have the same weight, then BFQ - guarantees the expected throughput distribution without ever idling - the device. It uses preemption instead. Throughput is then much - higher in this common scenario. - -- ioprio classes are served in strict priority order, i.e., - lower-priority queues are not served as long as there are - higher-priority queues. Among queues in the same class, the - bandwidth is distributed in proportion to the weight of each - queue. A very thin extra bandwidth is however guaranteed to - the Idle class, to prevent it from starving. - - -3. What are BFQ's tunables and how to properly configure BFQ? -============================================================= - -Most BFQ tunables affect service guarantees (basically latency and -fairness) and throughput. For full details on how to choose the -desired tradeoff between service guarantees and throughput, see the -parameters slice_idle, strict_guarantees and low_latency. For details -on how to maximise throughput, see slice_idle, timeout_sync and -max_budget. The other performance-related parameters have been -inherited from, and have been preserved mostly for compatibility with -CFQ. So far, no performance improvement has been reported after -changing the latter parameters in BFQ. - -In particular, the tunables back_seek-max, back_seek_penalty, -fifo_expire_async and fifo_expire_sync below are the same as in -CFQ. Their description is just copied from that for CFQ. Some -considerations in the description of slice_idle are copied from CFQ -too. - -per-process ioprio and weight ------------------------------ - -Unless the cgroups interface is used (see "4. BFQ group scheduling"), -weights can be assigned to processes only indirectly, through I/O -priorities, and according to the relation: -weight = (IOPRIO_BE_NR - ioprio) * 10. - -Beware that, if low-latency is set, then BFQ automatically raises the -weight of the queues associated with interactive and soft real-time -applications. Unset this tunable if you need/want to control weights. - -slice_idle ----------- - -This parameter specifies how long BFQ should idle for next I/O -request, when certain sync BFQ queues become empty. By default -slice_idle is a non-zero value. Idling has a double purpose: boosting -throughput and making sure that the desired throughput distribution is -respected (see the description of how BFQ works, and, if needed, the -papers referred there). - -As for throughput, idling can be very helpful on highly seeky media -like single spindle SATA/SAS disks where we can cut down on overall -number of seeks and see improved throughput. - -Setting slice_idle to 0 will remove all the idling on queues and one -should see an overall improved throughput on faster storage devices -like multiple SATA/SAS disks in hardware RAID configuration, as well -as flash-based storage with internal command queueing (and -parallelism). - -So depending on storage and workload, it might be useful to set -slice_idle=0. In general for SATA/SAS disks and software RAID of -SATA/SAS disks keeping slice_idle enabled should be useful. For any -configurations where there are multiple spindles behind single LUN -(Host based hardware RAID controller or for storage arrays), or with -flash-based fast storage, setting slice_idle=0 might end up in better -throughput and acceptable latencies. - -Idling is however necessary to have service guarantees enforced in -case of differentiated weights or differentiated I/O-request lengths. -To see why, suppose that a given BFQ queue A must get several I/O -requests served for each request served for another queue B. Idling -ensures that, if A makes a new I/O request slightly after becoming -empty, then no request of B is dispatched in the middle, and thus A -does not lose the possibility to get more than one request dispatched -before the next request of B is dispatched. Note that idling -guarantees the desired differentiated treatment of queues only in -terms of I/O-request dispatches. To guarantee that the actual service -order then corresponds to the dispatch order, the strict_guarantees -tunable must be set too. - -There is an important flipside for idling: apart from the above cases -where it is beneficial also for throughput, idling can severely impact -throughput. One important case is random workload. Because of this -issue, BFQ tends to avoid idling as much as possible, when it is not -beneficial also for throughput (as detailed in Section 2). As a -consequence of this behavior, and of further issues described for the -strict_guarantees tunable, short-term service guarantees may be -occasionally violated. And, in some cases, these guarantees may be -more important than guaranteeing maximum throughput. For example, in -video playing/streaming, a very low drop rate may be more important -than maximum throughput. In these cases, consider setting the -strict_guarantees parameter. - -slice_idle_us -------------- - -Controls the same tuning parameter as slice_idle, but in microseconds. -Either tunable can be used to set idling behavior. Afterwards, the -other tunable will reflect the newly set value in sysfs. - -strict_guarantees ------------------ - -If this parameter is set (default: unset), then BFQ - -- always performs idling when the in-service queue becomes empty; - -- forces the device to serve one I/O request at a time, by dispatching a - new request only if there is no outstanding request. - -In the presence of differentiated weights or I/O-request sizes, both -the above conditions are needed to guarantee that every BFQ queue -receives its allotted share of the bandwidth. The first condition is -needed for the reasons explained in the description of the slice_idle -tunable. The second condition is needed because all modern storage -devices reorder internally-queued requests, which may trivially break -the service guarantees enforced by the I/O scheduler. - -Setting strict_guarantees may evidently affect throughput. - -back_seek_max -------------- - -This specifies, given in Kbytes, the maximum "distance" for backward seeking. -The distance is the amount of space from the current head location to the -sectors that are backward in terms of distance. - -This parameter allows the scheduler to anticipate requests in the "backward" -direction and consider them as being the "next" if they are within this -distance from the current head location. - -back_seek_penalty ------------------ - -This parameter is used to compute the cost of backward seeking. If the -backward distance of request is just 1/back_seek_penalty from a "front" -request, then the seeking cost of two requests is considered equivalent. - -So scheduler will not bias toward one or the other request (otherwise scheduler -will bias toward front request). Default value of back_seek_penalty is 2. - -fifo_expire_async ------------------ - -This parameter is used to set the timeout of asynchronous requests. Default -value of this is 248ms. - -fifo_expire_sync ----------------- - -This parameter is used to set the timeout of synchronous requests. Default -value of this is 124ms. In case to favor synchronous requests over asynchronous -one, this value should be decreased relative to fifo_expire_async. - -low_latency ------------ - -This parameter is used to enable/disable BFQ's low latency mode. By -default, low latency mode is enabled. If enabled, interactive and soft -real-time applications are privileged and experience a lower latency, -as explained in more detail in the description of how BFQ works. - -DISABLE this mode if you need full control on bandwidth -distribution. In fact, if it is enabled, then BFQ automatically -increases the bandwidth share of privileged applications, as the main -means to guarantee a lower latency to them. - -In addition, as already highlighted at the beginning of this document, -DISABLE this mode if your only goal is to achieve a high throughput. -In fact, privileging the I/O of some application over the rest may -entail a lower throughput. To achieve the highest-possible throughput -on a non-rotational device, setting slice_idle to 0 may be needed too -(at the cost of giving up any strong guarantee on fairness and low -latency). - -timeout_sync ------------- - -Maximum amount of device time that can be given to a task (queue) once -it has been selected for service. On devices with costly seeks, -increasing this time usually increases maximum throughput. On the -opposite end, increasing this time coarsens the granularity of the -short-term bandwidth and latency guarantees, especially if the -following parameter is set to zero. - -max_budget ----------- - -Maximum amount of service, measured in sectors, that can be provided -to a BFQ queue once it is set in service (of course within the limits -of the above timeout). According to what said in the description of -the algorithm, larger values increase the throughput in proportion to -the percentage of sequential I/O requests issued. The price of larger -values is that they coarsen the granularity of short-term bandwidth -and latency guarantees. - -The default value is 0, which enables auto-tuning: BFQ sets max_budget -to the maximum number of sectors that can be served during -timeout_sync, according to the estimated peak rate. - -For specific devices, some users have occasionally reported to have -reached a higher throughput by setting max_budget explicitly, i.e., by -setting max_budget to a higher value than 0. In particular, they have -set max_budget to higher values than those to which BFQ would have set -it with auto-tuning. An alternative way to achieve this goal is to -just increase the value of timeout_sync, leaving max_budget equal to 0. - -weights -------- - -Read-only parameter, used to show the weights of the currently active -BFQ queues. - - -4. Group scheduling with BFQ -============================ - -BFQ supports both cgroups-v1 and cgroups-v2 io controllers, namely -blkio and io. In particular, BFQ supports weight-based proportional -share. To activate cgroups support, set BFQ_GROUP_IOSCHED. - -4-1 Service guarantees provided -------------------------------- - -With BFQ, proportional share means true proportional share of the -device bandwidth, according to group weights. For example, a group -with weight 200 gets twice the bandwidth, and not just twice the time, -of a group with weight 100. - -BFQ supports hierarchies (group trees) of any depth. Bandwidth is -distributed among groups and processes in the expected way: for each -group, the children of the group share the whole bandwidth of the -group in proportion to their weights. In particular, this implies -that, for each leaf group, every process of the group receives the -same share of the whole group bandwidth, unless the ioprio of the -process is modified. - -The resource-sharing guarantee for a group may partially or totally -switch from bandwidth to time, if providing bandwidth guarantees to -the group lowers the throughput too much. This switch occurs on a -per-process basis: if a process of a leaf group causes throughput loss -if served in such a way to receive its share of the bandwidth, then -BFQ switches back to just time-based proportional share for that -process. - -4-2 Interface -------------- - -To get proportional sharing of bandwidth with BFQ for a given device, -BFQ must of course be the active scheduler for that device. - -Within each group directory, the names of the files associated with -BFQ-specific cgroup parameters and stats begin with the "bfq." -prefix. So, with cgroups-v1 or cgroups-v2, the full prefix for -BFQ-specific files is "blkio.bfq." or "io.bfq." For example, the group -parameter to set the weight of a group with BFQ is blkio.bfq.weight -or io.bfq.weight. - -As for cgroups-v1 (blkio controller), the exact set of stat files -created, and kept up-to-date by bfq, depends on whether -CONFIG_BFQ_CGROUP_DEBUG is set. If it is set, then bfq creates all -the stat files documented in -Documentation/cgroup-v1/blkio-controller.rst. If, instead, -CONFIG_BFQ_CGROUP_DEBUG is not set, then bfq creates only the files -blkio.bfq.io_service_bytes -blkio.bfq.io_service_bytes_recursive -blkio.bfq.io_serviced -blkio.bfq.io_serviced_recursive - -The value of CONFIG_BFQ_CGROUP_DEBUG greatly influences the maximum -throughput sustainable with bfq, because updating the blkio.bfq.* -stats is rather costly, especially for some of the stats enabled by -CONFIG_BFQ_CGROUP_DEBUG. - -Parameters to set ------------------ - -For each group, there is only the following parameter to set. - -weight (namely blkio.bfq.weight or io.bfq-weight): the weight of the -group inside its parent. Available values: 1..10000 (default 100). The -linear mapping between ioprio and weights, described at the beginning -of the tunable section, is still valid, but all weights higher than -IOPRIO_BE_NR*10 are mapped to ioprio 0. - -Recall that, if low-latency is set, then BFQ automatically raises the -weight of the queues associated with interactive and soft real-time -applications. Unset this tunable if you need/want to control weights. - - -[1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O - Scheduler", Proceedings of the First Workshop on Mobile System - Technologies (MST-2015), May 2015. - http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf - -[2] P. Valente and M. Andreolini, "Improving Application - Responsiveness with the BFQ Disk I/O Scheduler", Proceedings of - the 5th Annual International Systems and Storage Conference - (SYSTOR '12), June 2012. - Slightly extended version: - http://algogroup.unimore.it/people/paolo/disk_sched/bfq-v1-suite- - results.pdf - -[3] https://github.com/Algodev-github/S diff --git a/Documentation/block/biodoc.rst b/Documentation/block/biodoc.rst new file mode 100644 index 000000000000..d6e30b680405 --- /dev/null +++ b/Documentation/block/biodoc.rst @@ -0,0 +1,1168 @@ +===================================================== +Notes on the Generic Block Layer Rewrite in Linux 2.5 +===================================================== + +.. note:: + + It seems that there are lot of outdated stuff here. This seems + to be written somewhat as a task list. Yet, eventually, something + here might still be useful. + +Notes Written on Jan 15, 2002: + - Jens Axboe + - Suparna Bhattacharya + +Last Updated May 2, 2002 + +September 2003: Updated I/O Scheduler portions + - Nick Piggin + +Introduction +============ + +These are some notes describing some aspects of the 2.5 block layer in the +context of the bio rewrite. The idea is to bring out some of the key +changes and a glimpse of the rationale behind those changes. + +Please mail corrections & suggestions to suparna@in.ibm.com. + +Credits +======= + +2.5 bio rewrite: + - Jens Axboe + +Many aspects of the generic block layer redesign were driven by and evolved +over discussions, prior patches and the collective experience of several +people. See sections 8 and 9 for a list of some related references. + +The following people helped with review comments and inputs for this +document: + + - Christoph Hellwig + - Arjan van de Ven + - Randy Dunlap + - Andre Hedrick + +The following people helped with fixes/contributions to the bio patches +while it was still work-in-progress: + + - David S. Miller + + +.. Description of Contents: + + 1. Scope for tuning of logic to various needs + 1.1 Tuning based on device or low level driver capabilities + - Per-queue parameters + - Highmem I/O support + - I/O scheduler modularization + 1.2 Tuning based on high level requirements/capabilities + 1.2.1 Request Priority/Latency + 1.3 Direct access/bypass to lower layers for diagnostics and special + device operations + 1.3.1 Pre-built commands + 2. New flexible and generic but minimalist i/o structure or descriptor + (instead of using buffer heads at the i/o layer) + 2.1 Requirements/Goals addressed + 2.2 The bio struct in detail (multi-page io unit) + 2.3 Changes in the request structure + 3. Using bios + 3.1 Setup/teardown (allocation, splitting) + 3.2 Generic bio helper routines + 3.2.1 Traversing segments and completion units in a request + 3.2.2 Setting up DMA scatterlists + 3.2.3 I/O completion + 3.2.4 Implications for drivers that do not interpret bios (don't handle + multiple segments) + 3.3 I/O submission + 4. The I/O scheduler + 5. Scalability related changes + 5.1 Granular locking: Removal of io_request_lock + 5.2 Prepare for transition to 64 bit sector_t + 6. Other Changes/Implications + 6.1 Partition re-mapping handled by the generic block layer + 7. A few tips on migration of older drivers + 8. A list of prior/related/impacted patches/ideas + 9. Other References/Discussion Threads + + +Bio Notes +========= + +Let us discuss the changes in the context of how some overall goals for the +block layer are addressed. + +1. Scope for tuning the generic logic to satisfy various requirements +===================================================================== + +The block layer design supports adaptable abstractions to handle common +processing with the ability to tune the logic to an appropriate extent +depending on the nature of the device and the requirements of the caller. +One of the objectives of the rewrite was to increase the degree of tunability +and to enable higher level code to utilize underlying device/driver +capabilities to the maximum extent for better i/o performance. This is +important especially in the light of ever improving hardware capabilities +and application/middleware software designed to take advantage of these +capabilities. + +1.1 Tuning based on low level device / driver capabilities +---------------------------------------------------------- + +Sophisticated devices with large built-in caches, intelligent i/o scheduling +optimizations, high memory DMA support, etc may find some of the +generic processing an overhead, while for less capable devices the +generic functionality is essential for performance or correctness reasons. +Knowledge of some of the capabilities or parameters of the device should be +used at the generic block layer to take the right decisions on +behalf of the driver. + +How is this achieved ? + +Tuning at a per-queue level: + +i. Per-queue limits/values exported to the generic layer by the driver + +Various parameters that the generic i/o scheduler logic uses are set at +a per-queue level (e.g maximum request size, maximum number of segments in +a scatter-gather list, logical block size) + +Some parameters that were earlier available as global arrays indexed by +major/minor are now directly associated with the queue. Some of these may +move into the block device structure in the future. Some characteristics +have been incorporated into a queue flags field rather than separate fields +in themselves. There are blk_queue_xxx functions to set the parameters, +rather than update the fields directly + +Some new queue property settings: + + blk_queue_bounce_limit(q, u64 dma_address) + Enable I/O to highmem pages, dma_address being the + limit. No highmem default. + + blk_queue_max_sectors(q, max_sectors) + Sets two variables that limit the size of the request. + + - The request queue's max_sectors, which is a soft size in + units of 512 byte sectors, and could be dynamically varied + by the core kernel. + + - The request queue's max_hw_sectors, which is a hard limit + and reflects the maximum size request a driver can handle + in units of 512 byte sectors. + + The default for both max_sectors and max_hw_sectors is + 255. The upper limit of max_sectors is 1024. + + blk_queue_max_phys_segments(q, max_segments) + Maximum physical segments you can handle in a request. 128 + default (driver limit). (See 3.2.2) + + blk_queue_max_hw_segments(q, max_segments) + Maximum dma segments the hardware can handle in a request. 128 + default (host adapter limit, after dma remapping). + (See 3.2.2) + + blk_queue_max_segment_size(q, max_seg_size) + Maximum size of a clustered segment, 64kB default. + + blk_queue_logical_block_size(q, logical_block_size) + Lowest possible sector size that the hardware can operate + on, 512 bytes default. + +New queue flags: + + QUEUE_FLAG_CLUSTER (see 3.2.2) + QUEUE_FLAG_QUEUED (see 3.2.4) + + +ii. High-mem i/o capabilities are now considered the default + +The generic bounce buffer logic, present in 2.4, where the block layer would +by default copyin/out i/o requests on high-memory buffers to low-memory buffers +assuming that the driver wouldn't be able to handle it directly, has been +changed in 2.5. The bounce logic is now applied only for memory ranges +for which the device cannot handle i/o. A driver can specify this by +setting the queue bounce limit for the request queue for the device +(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out +where a device is capable of handling high memory i/o. + +In order to enable high-memory i/o where the device is capable of supporting +it, the pci dma mapping routines and associated data structures have now been +modified to accomplish a direct page -> bus translation, without requiring +a virtual address mapping (unlike the earlier scheme of virtual address +-> bus translation). So this works uniformly for high-memory pages (which +do not have a corresponding kernel virtual address space mapping) and +low-memory pages. + +Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion +on PCI high mem DMA aspects and mapping of scatter gather lists, and support +for 64 bit PCI. + +Special handling is required only for cases where i/o needs to happen on +pages at physical memory addresses beyond what the device can support. In these +cases, a bounce bio representing a buffer from the supported memory range +is used for performing the i/o with copyin/copyout as needed depending on +the type of the operation. For example, in case of a read operation, the +data read has to be copied to the original buffer on i/o completion, so a +callback routine is set up to do this, while for write, the data is copied +from the original buffer to the bounce buffer prior to issuing the +operation. Since an original buffer may be in a high memory area that's not +mapped in kernel virtual addr, a kmap operation may be required for +performing the copy, and special care may be needed in the completion path +as it may not be in irq context. Special care is also required (by way of +GFP flags) when allocating bounce buffers, to avoid certain highmem +deadlock possibilities. + +It is also possible that a bounce buffer may be allocated from high-memory +area that's not mapped in kernel virtual addr, but within the range that the +device can use directly; so the bounce page may need to be kmapped during +copy operations. [Note: This does not hold in the current implementation, +though] + +There are some situations when pages from high memory may need to +be kmapped, even if bounce buffers are not necessary. For example a device +may need to abort DMA operations and revert to PIO for the transfer, in +which case a virtual mapping of the page is required. For SCSI it is also +done in some scenarios where the low level driver cannot be trusted to +handle a single sg entry correctly. The driver is expected to perform the +kmaps as needed on such occasions as appropriate. A driver could also use +the blk_queue_bounce() routine on its own to bounce highmem i/o to low +memory for specific requests if so desired. + +iii. The i/o scheduler algorithm itself can be replaced/set as appropriate + +As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular +queue or pick from (copy) existing generic schedulers and replace/override +certain portions of it. The 2.5 rewrite provides improved modularization +of the i/o scheduler. There are more pluggable callbacks, e.g for init, +add request, extract request, which makes it possible to abstract specific +i/o scheduling algorithm aspects and details outside of the generic loop. +It also makes it possible to completely hide the implementation details of +the i/o scheduler from block drivers. + +I/O scheduler wrappers are to be used instead of accessing the queue directly. +See section 4. The I/O scheduler for details. + +1.2 Tuning Based on High level code capabilities +------------------------------------------------ + +i. Application capabilities for raw i/o + +This comes from some of the high-performance database/middleware +requirements where an application prefers to make its own i/o scheduling +decisions based on an understanding of the access patterns and i/o +characteristics + +ii. High performance filesystems or other higher level kernel code's +capabilities + +Kernel components like filesystems could also take their own i/o scheduling +decisions for optimizing performance. Journalling filesystems may need +some control over i/o ordering. + +What kind of support exists at the generic block layer for this ? + +The flags and rw fields in the bio structure can be used for some tuning +from above e.g indicating that an i/o is just a readahead request, or priority +settings (currently unused). As far as user applications are concerned they +would need an additional mechanism either via open flags or ioctls, or some +other upper level mechanism to communicate such settings to block. + +1.2.1 Request Priority/Latency +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Todo/Under discussion:: + + Arjan's proposed request priority scheme allows higher levels some broad + control (high/med/low) over the priority of an i/o request vs other pending + requests in the queue. For example it allows reads for bringing in an + executable page on demand to be given a higher priority over pending write + requests which haven't aged too much on the queue. Potentially this priority + could even be exposed to applications in some manner, providing higher level + tunability. Time based aging avoids starvation of lower priority + requests. Some bits in the bi_opf flags field in the bio structure are + intended to be used for this priority information. + + +1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode) +----------------------------------------------------------------------- + +(e.g Diagnostics, Systems Management) + +There are situations where high-level code needs to have direct access to +the low level device capabilities or requires the ability to issue commands +to the device bypassing some of the intermediate i/o layers. +These could, for example, be special control commands issued through ioctl +interfaces, or could be raw read/write commands that stress the drive's +capabilities for certain kinds of fitness tests. Having direct interfaces at +multiple levels without having to pass through upper layers makes +it possible to perform bottom up validation of the i/o path, layer by +layer, starting from the media. + +The normal i/o submission interfaces, e.g submit_bio, could be bypassed +for specially crafted requests which such ioctl or diagnostics +interfaces would typically use, and the elevator add_request routine +can instead be used to directly insert such requests in the queue or preferably +the blk_do_rq routine can be used to place the request on the queue and +wait for completion. Alternatively, sometimes the caller might just +invoke a lower level driver specific interface with the request as a +parameter. + +If the request is a means for passing on special information associated with +the command, then such information is associated with the request->special +field (rather than misuse the request->buffer field which is meant for the +request data buffer's virtual mapping). + +For passing request data, the caller must build up a bio descriptor +representing the concerned memory buffer if the underlying driver interprets +bio segments or uses the block layer end*request* functions for i/o +completion. Alternatively one could directly use the request->buffer field to +specify the virtual address of the buffer, if the driver expects buffer +addresses passed in this way and ignores bio entries for the request type +involved. In the latter case, the driver would modify and manage the +request->buffer, request->sector and request->nr_sectors or +request->current_nr_sectors fields itself rather than using the block layer +end_request or end_that_request_first completion interfaces. +(See 2.3 or Documentation/block/request.rst for a brief explanation of +the request structure fields) + +:: + + [TBD: end_that_request_last should be usable even in this case; + Perhaps an end_that_direct_request_first routine could be implemented to make + handling direct requests easier for such drivers; Also for drivers that + expect bios, a helper function could be provided for setting up a bio + corresponding to a data buffer] + + + + + +1.3.1 Pre-built Commands +^^^^^^^^^^^^^^^^^^^^^^^^ + +A request can be created with a pre-built custom command to be sent directly +to the device. The cmd block in the request structure has room for filling +in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for +command pre-building, and the type of the request is now indicated +through rq->flags instead of via rq->cmd) + +The request structure flags can be set up to indicate the type of request +in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC: +packet command issued via blk_do_rq, REQ_SPECIAL: special request). + +It can help to pre-build device commands for requests in advance. +Drivers can now specify a request prepare function (q->prep_rq_fn) that the +block layer would invoke to pre-build device commands for a given request, +or perform other preparatory processing for the request. This is routine is +called by elv_next_request(), i.e. typically just before servicing a request. +(The prepare function would not be called for requests that have RQF_DONTPREP +enabled) + +Aside: + Pre-building could possibly even be done early, i.e before placing the + request on the queue, rather than construct the command on the fly in the + driver while servicing the request queue when it may affect latencies in + interrupt context or responsiveness in general. One way to add early + pre-building would be to do it whenever we fail to merge on a request. + Now REQ_NOMERGE is set in the request flags to skip this one in the future, + which means that it will not change before we feed it to the device. So + the pre-builder hook can be invoked there. + + +2. Flexible and generic but minimalist i/o structure/descriptor +=============================================================== + +2.1 Reason for a new structure and requirements addressed +--------------------------------------------------------- + +Prior to 2.5, buffer heads were used as the unit of i/o at the generic block +layer, and the low level request structure was associated with a chain of +buffer heads for a contiguous i/o request. This led to certain inefficiencies +when it came to large i/o requests and readv/writev style operations, as it +forced such requests to be broken up into small chunks before being passed +on to the generic block layer, only to be merged by the i/o scheduler +when the underlying device was capable of handling the i/o in one shot. +Also, using the buffer head as an i/o structure for i/os that didn't originate +from the buffer cache unnecessarily added to the weight of the descriptors +which were generated for each such chunk. + +The following were some of the goals and expectations considered in the +redesign of the block i/o data structure in 2.5. + +1. Should be appropriate as a descriptor for both raw and buffered i/o - + avoid cache related fields which are irrelevant in the direct/page i/o path, + or filesystem block size alignment restrictions which may not be relevant + for raw i/o. +2. Ability to represent high-memory buffers (which do not have a virtual + address mapping in kernel address space). +3. Ability to represent large i/os w/o unnecessarily breaking them up (i.e + greater than PAGE_SIZE chunks in one shot) +4. At the same time, ability to retain independent identity of i/os from + different sources or i/o units requiring individual completion (e.g. for + latency reasons) +5. Ability to represent an i/o involving multiple physical memory segments + (including non-page aligned page fragments, as specified via readv/writev) + without unnecessarily breaking it up, if the underlying device is capable of + handling it. +6. Preferably should be based on a memory descriptor structure that can be + passed around different types of subsystems or layers, maybe even + networking, without duplication or extra copies of data/descriptor fields + themselves in the process +7. Ability to handle the possibility of splits/merges as the structure passes + through layered drivers (lvm, md, evms), with minimal overhead. + +The solution was to define a new structure (bio) for the block layer, +instead of using the buffer head structure (bh) directly, the idea being +avoidance of some associated baggage and limitations. The bio structure +is uniformly used for all i/o at the block layer ; it forms a part of the +bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are +mapped to bio structures. + +2.2 The bio struct +------------------ + +The bio structure uses a vector representation pointing to an array of tuples +of to describe the i/o buffer, and has various other +fields describing i/o parameters and state that needs to be maintained for +performing the i/o. + +Notice that this representation means that a bio has no virtual address +mapping at all (unlike buffer heads). + +:: + + struct bio_vec { + struct page *bv_page; + unsigned short bv_len; + unsigned short bv_offset; + }; + + /* + * main unit of I/O for the block layer and lower layers (ie drivers) + */ + struct bio { + struct bio *bi_next; /* request queue link */ + struct block_device *bi_bdev; /* target device */ + unsigned long bi_flags; /* status, command, etc */ + unsigned long bi_opf; /* low bits: r/w, high: priority */ + + unsigned int bi_vcnt; /* how may bio_vec's */ + struct bvec_iter bi_iter; /* current index into bio_vec array */ + + unsigned int bi_size; /* total size in bytes */ + unsigned short bi_hw_segments; /* segments after DMA remapping */ + unsigned int bi_max; /* max bio_vecs we can hold + used as index into pool */ + struct bio_vec *bi_io_vec; /* the actual vec list */ + bio_end_io_t *bi_end_io; /* bi_end_io (bio) */ + atomic_t bi_cnt; /* pin count: free when it hits zero */ + void *bi_private; + }; + +With this multipage bio design: + +- Large i/os can be sent down in one go using a bio_vec list consisting + of an array of fragments (similar to the way fragments + are represented in the zero-copy network code) +- Splitting of an i/o request across multiple devices (as in the case of + lvm or raid) is achieved by cloning the bio (where the clone points to + the same bi_io_vec array, but with the index and size accordingly modified) +- A linked list of bios is used as before for unrelated merges [*]_ - this + avoids reallocs and makes independent completions easier to handle. +- Code that traverses the req list can find all the segments of a bio + by using rq_for_each_segment. This handles the fact that a request + has multiple bios, each of which can have multiple segments. +- Drivers which can't process a large bio in one shot can use the bi_iter + field to keep track of the next bio_vec entry to process. + (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE) + [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying + bi_offset an len fields] + +.. [*] + + unrelated merges -- a request ends up containing two or more bios that + didn't originate from the same place. + +bi_end_io() i/o callback gets called on i/o completion of the entire bio. + +At a lower level, drivers build a scatter gather list from the merged bios. +The scatter gather list is in the form of an array of +entries with their corresponding dma address mappings filled in at the +appropriate time. As an optimization, contiguous physical pages can be +covered by a single entry where refers to the first page and +covers the range of pages (up to 16 contiguous pages could be covered this +way). There is a helper routine (blk_rq_map_sg) which drivers can use to build +the sg list. + +Note: Right now the only user of bios with more than one page is ll_rw_kio, +which in turn means that only raw I/O uses it (direct i/o may not work +right now). The intent however is to enable clustering of pages etc to +become possible. The pagebuf abstraction layer from SGI also uses multi-page +bios, but that is currently not included in the stock development kernels. +The same is true of Andrew Morton's work-in-progress multipage bio writeout +and readahead patches. + +2.3 Changes in the Request Structure +------------------------------------ + +The request structure is the structure that gets passed down to low level +drivers. The block layer make_request function builds up a request structure, +places it on the queue and invokes the drivers request_fn. The driver makes +use of block layer helper routine elv_next_request to pull the next request +off the queue. Control or diagnostic functions might bypass block and directly +invoke underlying driver entry points passing in a specially constructed +request structure. + +Only some relevant fields (mainly those which changed or may be referred +to in some of the discussion here) are listed below, not necessarily in +the order in which they occur in the structure (see include/linux/blkdev.h) +Refer to Documentation/block/request.rst for details about all the request +structure fields and a quick reference about the layers which are +supposed to use or modify those fields:: + + struct request { + struct list_head queuelist; /* Not meant to be directly accessed by + the driver. + Used by q->elv_next_request_fn + rq->queue is gone + */ + . + . + unsigned char cmd[16]; /* prebuilt command data block */ + unsigned long flags; /* also includes earlier rq->cmd settings */ + . + . + sector_t sector; /* this field is now of type sector_t instead of int + preparation for 64 bit sectors */ + . + . + + /* Number of scatter-gather DMA addr+len pairs after + * physical address coalescing is performed. + */ + unsigned short nr_phys_segments; + + /* Number of scatter-gather addr+len pairs after + * physical and DMA remapping hardware coalescing is performed. + * This is the number of scatter-gather entries the driver + * will actually have to deal with after DMA mapping is done. + */ + unsigned short nr_hw_segments; + + /* Various sector counts */ + unsigned long nr_sectors; /* no. of sectors left: driver modifiable */ + unsigned long hard_nr_sectors; /* block internal copy of above */ + unsigned int current_nr_sectors; /* no. of sectors left in the + current segment:driver modifiable */ + unsigned long hard_cur_sectors; /* block internal copy of the above */ + . + . + int tag; /* command tag associated with request */ + void *special; /* same as before */ + char *buffer; /* valid only for low memory buffers up to + current_nr_sectors */ + . + . + struct bio *bio, *biotail; /* bio list instead of bh */ + struct request_list *rl; + } + +See the req_ops and req_flag_bits definitions for an explanation of the various +flags available. Some bits are used by the block layer or i/o scheduler. + +The behaviour of the various sector counts are almost the same as before, +except that since we have multi-segment bios, current_nr_sectors refers +to the numbers of sectors in the current segment being processed which could +be one of the many segments in the current bio (i.e i/o completion unit). +The nr_sectors value refers to the total number of sectors in the whole +request that remain to be transferred (no change). The purpose of the +hard_xxx values is for block to remember these counts every time it hands +over the request to the driver. These values are updated by block on +end_that_request_first, i.e. every time the driver completes a part of the +transfer and invokes block end*request helpers to mark this. The +driver should not modify these values. The block layer sets up the +nr_sectors and current_nr_sectors fields (based on the corresponding +hard_xxx values and the number of bytes transferred) and updates it on +every transfer that invokes end_that_request_first. It does the same for the +buffer, bio, bio->bi_iter fields too. + +The buffer field is just a virtual address mapping of the current segment +of the i/o buffer in cases where the buffer resides in low-memory. For high +memory i/o, this field is not valid and must not be used by drivers. + +Code that sets up its own request structures and passes them down to +a driver needs to be careful about interoperation with the block layer helper +functions which the driver uses. (Section 1.3) + +3. Using bios +============= + +3.1 Setup/Teardown +------------------ + +There are routines for managing the allocation, and reference counting, and +freeing of bios (bio_alloc, bio_get, bio_put). + +This makes use of Ingo Molnar's mempool implementation, which enables +subsystems like bio to maintain their own reserve memory pools for guaranteed +deadlock-free allocations during extreme VM load. For example, the VM +subsystem makes use of the block layer to writeout dirty pages in order to be +able to free up memory space, a case which needs careful handling. The +allocation logic draws from the preallocated emergency reserve in situations +where it cannot allocate through normal means. If the pool is empty and it +can wait, then it would trigger action that would help free up memory or +replenish the pool (without deadlocking) and wait for availability in the pool. +If it is in IRQ context, and hence not in a position to do this, allocation +could fail if the pool is empty. In general mempool always first tries to +perform allocation without having to wait, even if it means digging into the +pool as long it is not less that 50% full. + +On a free, memory is released to the pool or directly freed depending on +the current availability in the pool. The mempool interface lets the +subsystem specify the routines to be used for normal alloc and free. In the +case of bio, these routines make use of the standard slab allocator. + +The caller of bio_alloc is expected to taken certain steps to avoid +deadlocks, e.g. avoid trying to allocate more memory from the pool while +already holding memory obtained from the pool. + +:: + + [TBD: This is a potential issue, though a rare possibility + in the bounce bio allocation that happens in the current code, since + it ends up allocating a second bio from the same pool while + holding the original bio ] + +Memory allocated from the pool should be released back within a limited +amount of time (in the case of bio, that would be after the i/o is completed). +This ensures that if part of the pool has been used up, some work (in this +case i/o) must already be in progress and memory would be available when it +is over. If allocating from multiple pools in the same code path, the order +or hierarchy of allocation needs to be consistent, just the way one deals +with multiple locks. + +The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc()) +for a non-clone bio. There are the 6 pools setup for different size biovecs, +so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the +given size from these slabs. + +The bio_get() routine may be used to hold an extra reference on a bio prior +to i/o submission, if the bio fields are likely to be accessed after the +i/o is issued (since the bio may otherwise get freed in case i/o completion +happens in the meantime). + +The bio_clone_fast() routine may be used to duplicate a bio, where the clone +shares the bio_vec_list with the original bio (i.e. both point to the +same bio_vec_list). This would typically be used for splitting i/o requests +in lvm or md. + +3.2 Generic bio helper Routines +------------------------------- + +3.2.1 Traversing segments and completion units in a request +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The macro rq_for_each_segment() should be used for traversing the bios +in the request list (drivers should avoid directly trying to do it +themselves). Using these helpers should also make it easier to cope +with block changes in the future. + +:: + + struct req_iterator iter; + rq_for_each_segment(bio_vec, rq, iter) + /* bio_vec is now current segment */ + +I/O completion callbacks are per-bio rather than per-segment, so drivers +that traverse bio chains on completion need to keep that in mind. Drivers +which don't make a distinction between segments and completion units would +need to be reorganized to support multi-segment bios. + +3.2.2 Setting up DMA scatterlists +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The blk_rq_map_sg() helper routine would be used for setting up scatter +gather lists from a request, so a driver need not do it on its own. + + nr_segments = blk_rq_map_sg(q, rq, scatterlist); + +The helper routine provides a level of abstraction which makes it easier +to modify the internals of request to scatterlist conversion down the line +without breaking drivers. The blk_rq_map_sg routine takes care of several +things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER +is set) and correct segment accounting to avoid exceeding the limits which +the i/o hardware can handle, based on various queue properties. + +- Prevents a clustered segment from crossing a 4GB mem boundary +- Avoids building segments that would exceed the number of physical + memory segments that the driver can handle (phys_segments) and the + number that the underlying hardware can handle at once, accounting for + DMA remapping (hw_segments) (i.e. IOMMU aware limits). + +Routines which the low level driver can use to set up the segment limits: + +blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of +hw data segments in a request (i.e. the maximum number of address/length +pairs the host adapter can actually hand to the device at once) + +blk_queue_max_phys_segments() : Sets an upper limit on the maximum number +of physical data segments in a request (i.e. the largest sized scatter list +a driver could handle) + +3.2.3 I/O completion +^^^^^^^^^^^^^^^^^^^^ + +The existing generic block layer helper routines end_request, +end_that_request_first and end_that_request_last can be used for i/o +completion (and setting things up so the rest of the i/o or the next +request can be kicked of) as before. With the introduction of multi-page +bio support, end_that_request_first requires an additional argument indicating +the number of sectors completed. + +3.2.4 Implications for drivers that do not interpret bios +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +(don't handle multiple segments) + +Drivers that do not interpret bios e.g those which do not handle multiple +segments and do not support i/o into high memory addresses (require bounce +buffers) and expect only virtually mapped buffers, can access the rq->buffer +field. As before the driver should use current_nr_sectors to determine the +size of remaining data in the current segment (that is the maximum it can +transfer in one go unless it interprets segments), and rely on the block layer +end_request, or end_that_request_first/last to take care of all accounting +and transparent mapping of the next bio segment when a segment boundary +is crossed on completion of a transfer. (The end*request* functions should +be used if only if the request has come down from block/bio path, not for +direct access requests which only specify rq->buffer without a valid rq->bio) + +3.3 I/O Submission +------------------ + +The routine submit_bio() is used to submit a single io. Higher level i/o +routines make use of this: + +(a) Buffered i/o: + +The routine submit_bh() invokes submit_bio() on a bio corresponding to the +bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before. + +(b) Kiobuf i/o (for raw/direct i/o): + +The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and +maps the array to one or more multi-page bios, issuing submit_bio() to +perform the i/o on each of these. + +The embedded bh array in the kiobuf structure has been removed and no +preallocation of bios is done for kiobufs. [The intent is to remove the +blocks array as well, but it's currently in there to kludge around direct i/o.] +Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc. + +Todo/Observation: + + A single kiobuf structure is assumed to correspond to a contiguous range + of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec. + So right now it wouldn't work for direct i/o on non-contiguous blocks. + This is to be resolved. The eventual direction is to replace kiobuf + by kvec's. + + Badari Pulavarty has a patch to implement direct i/o correctly using + bio and kvec. + + +(c) Page i/o: + +Todo/Under discussion: + + Andrew Morton's multi-page bio patches attempt to issue multi-page + writeouts (and reads) from the page cache, by directly building up + large bios for submission completely bypassing the usage of buffer + heads. This work is still in progress. + + Christoph Hellwig had some code that uses bios for page-io (rather than + bh). This isn't included in bio as yet. Christoph was also working on a + design for representing virtual/real extents as an entity and modifying + some of the address space ops interfaces to utilize this abstraction rather + than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf + abstraction, but intended to be as lightweight as possible). + +(d) Direct access i/o: + +Direct access requests that do not contain bios would be submitted differently +as discussed earlier in section 1.3. + +Aside: + + Kvec i/o: + + Ben LaHaise's aio code uses a slightly different structure instead + of kiobufs, called a kvec_cb. This contains an array of + tuples (very much like the networking code), together with a callback function + and data pointer. This is embedded into a brw_cb structure when passed + to brw_kvec_async(). + + Now it should be possible to directly map these kvecs to a bio. Just as while + cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec + array pointer to point to the veclet array in kvecs. + + TBD: In order for this to work, some changes are needed in the way multi-page + bios are handled today. The values of the tuples in such a vector passed in + from higher level code should not be modified by the block layer in the course + of its request processing, since that would make it hard for the higher layer + to continue to use the vector descriptor (kvec) after i/o completes. Instead, + all such transient state should either be maintained in the request structure, + and passed on in some way to the endio completion routine. + + +4. The I/O scheduler +==================== + +I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch +queue and specific I/O schedulers. Unless stated otherwise, elevator is used +to refer to both parts and I/O scheduler to specific I/O schedulers. + +Block layer implements generic dispatch queue in `block/*.c`. +The generic dispatch queue is responsible for requeueing, handling non-fs +requests and all other subtleties. + +Specific I/O schedulers are responsible for ordering normal filesystem +requests. They can also choose to delay certain requests to improve +throughput or whatever purpose. As the plural form indicates, there are +multiple I/O schedulers. They can be built as modules but at least one should +be built inside the kernel. Each queue can choose different one and can also +change to another one dynamically. + +A block layer call to the i/o scheduler follows the convention elv_xxx(). This +calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx +and xxx might not match exactly, but use your imagination. If an elevator +doesn't implement a function, the switch does nothing or some minimal house +keeping work. + +4.1. I/O scheduler API +---------------------- + +The functions an elevator may implement are: (* are mandatory) + +=============================== ================================================ +elevator_merge_fn called to query requests for merge with a bio + +elevator_merge_req_fn called when two requests get merged. the one + which gets merged into the other one will be + never seen by I/O scheduler again. IOW, after + being merged, the request is gone. + +elevator_merged_fn called when a request in the scheduler has been + involved in a merge. It is used in the deadline + scheduler for example, to reposition the request + if its sorting order has changed. + +elevator_allow_merge_fn called whenever the block layer determines + that a bio can be merged into an existing + request safely. The io scheduler may still + want to stop a merge at this point if it + results in some sort of conflict internally, + this hook allows it to do that. Note however + that two *requests* can still be merged at later + time. Currently the io scheduler has no way to + prevent that. It can only learn about the fact + from elevator_merge_req_fn callback. + +elevator_dispatch_fn* fills the dispatch queue with ready requests. + I/O schedulers are free to postpone requests by + not filling the dispatch queue unless @force + is non-zero. Once dispatched, I/O schedulers + are not allowed to manipulate the requests - + they belong to generic dispatch queue. + +elevator_add_req_fn* called to add a new request into the scheduler + +elevator_former_req_fn +elevator_latter_req_fn These return the request before or after the + one specified in disk sort order. Used by the + block layer to find merge possibilities. + +elevator_completed_req_fn called when a request is completed. + +elevator_may_queue_fn returns true if the scheduler wants to allow the + current context to queue a new request even if + it is over the queue limit. This must be used + very carefully!! + +elevator_set_req_fn +elevator_put_req_fn Must be used to allocate and free any elevator + specific storage for a request. + +elevator_activate_req_fn Called when device driver first sees a request. + I/O schedulers can use this callback to + determine when actual execution of a request + starts. +elevator_deactivate_req_fn Called when device driver decides to delay + a request by requeueing it. + +elevator_init_fn* +elevator_exit_fn Allocate and free any elevator specific storage + for a queue. +=============================== ================================================ + +4.2 Request flows seen by I/O schedulers +---------------------------------------- + +All requests seen by I/O schedulers strictly follow one of the following three +flows. + + set_req_fn -> + + i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn -> + (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn + ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn + iii. [none] + + -> put_req_fn + +4.3 I/O scheduler implementation +-------------------------------- + +The generic i/o scheduler algorithm attempts to sort/merge/batch requests for +optimal disk scan and request servicing performance (based on generic +principles and device capabilities), optimized for: + +i. improved throughput +ii. improved latency +iii. better utilization of h/w & CPU time + +Characteristics: + +i. Binary tree +AS and deadline i/o schedulers use red black binary trees for disk position +sorting and searching, and a fifo linked list for time-based searching. This +gives good scalability and good availability of information. Requests are +almost always dispatched in disk sort order, so a cache is kept of the next +request in sort order to prevent binary tree lookups. + +This arrangement is not a generic block layer characteristic however, so +elevators may implement queues as they please. + +ii. Merge hash +AS and deadline use a hash table indexed by the last sector of a request. This +enables merging code to quickly look up "back merge" candidates, even when +multiple I/O streams are being performed at once on one disk. + +"Front merges", a new request being merged at the front of an existing request, +are far less common than "back merges" due to the nature of most I/O patterns. +Front merges are handled by the binary trees in AS and deadline schedulers. + +iii. Plugging the queue to batch requests in anticipation of opportunities for + merge/sort optimizations + +Plugging is an approach that the current i/o scheduling algorithm resorts to so +that it collects up enough requests in the queue to be able to take +advantage of the sorting/merging logic in the elevator. If the +queue is empty when a request comes in, then it plugs the request queue +(sort of like plugging the bath tub of a vessel to get fluid to build up) +till it fills up with a few more requests, before starting to service +the requests. This provides an opportunity to merge/sort the requests before +passing them down to the device. There are various conditions when the queue is +unplugged (to open up the flow again), either through a scheduled task or +could be on demand. For example wait_on_buffer sets the unplugging going +through sync_buffer() running blk_run_address_space(mapping). Or the caller +can do it explicity through blk_unplug(bdev). So in the read case, +the queue gets explicitly unplugged as part of waiting for completion on that +buffer. + +Aside: + This is kind of controversial territory, as it's not clear if plugging is + always the right thing to do. Devices typically have their own queues, + and allowing a big queue to build up in software, while letting the device be + idle for a while may not always make sense. The trick is to handle the fine + balance between when to plug and when to open up. Also now that we have + multi-page bios being queued in one shot, we may not need to wait to merge + a big request from the broken up pieces coming by. + +4.4 I/O contexts +---------------- + +I/O contexts provide a dynamically allocated per process data area. They may +be used in I/O schedulers, and in the block layer (could be used for IO statis, +priorities for example). See `*io_context` in block/ll_rw_blk.c, and as-iosched.c +for an example of usage in an i/o scheduler. + + +5. Scalability related changes +============================== + +5.1 Granular Locking: io_request_lock replaced by a per-queue lock +------------------------------------------------------------------ + +The global io_request_lock has been removed as of 2.5, to avoid +the scalability bottleneck it was causing, and has been replaced by more +granular locking. The request queue structure has a pointer to the +lock to be used for that queue. As a result, locking can now be +per-queue, with a provision for sharing a lock across queues if +necessary (e.g the scsi layer sets the queue lock pointers to the +corresponding adapter lock, which results in a per host locking +granularity). The locking semantics are the same, i.e. locking is +still imposed by the block layer, grabbing the lock before +request_fn execution which it means that lots of older drivers +should still be SMP safe. Drivers are free to drop the queue +lock themselves, if required. Drivers that explicitly used the +io_request_lock for serialization need to be modified accordingly. +Usually it's as easy as adding a global lock:: + + static DEFINE_SPINLOCK(my_driver_lock); + +and passing the address to that lock to blk_init_queue(). + +5.2 64 bit sector numbers (sector_t prepares for 64 bit support) +---------------------------------------------------------------- + +The sector number used in the bio structure has been changed to sector_t, +which could be defined as 64 bit in preparation for 64 bit sector support. + +6. Other Changes/Implications +============================= + +6.1 Partition re-mapping handled by the generic block layer +----------------------------------------------------------- + +In 2.5 some of the gendisk/partition related code has been reorganized. +Now the generic block layer performs partition-remapping early and thus +provides drivers with a sector number relative to whole device, rather than +having to take partition number into account in order to arrive at the true +sector number. The routine blk_partition_remap() is invoked by +generic_make_request even before invoking the queue specific make_request_fn, +so the i/o scheduler also gets to operate on whole disk sector numbers. This +should typically not require changes to block drivers, it just never gets +to invoke its own partition sector offset calculations since all bios +sent are offset from the beginning of the device. + + +7. A Few Tips on Migration of older drivers +=========================================== + +Old-style drivers that just use CURRENT and ignores clustered requests, +may not need much change. The generic layer will automatically handle +clustered requests, multi-page bios, etc for the driver. + +For a low performance driver or hardware that is PIO driven or just doesn't +support scatter-gather changes should be minimal too. + +The following are some points to keep in mind when converting old drivers +to bio. + +Drivers should use elv_next_request to pick up requests and are no longer +supposed to handle looping directly over the request list. +(struct request->queue has been removed) + +Now end_that_request_first takes an additional number_of_sectors argument. +It used to handle always just the first buffer_head in a request, now +it will loop and handle as many sectors (on a bio-segment granularity) +as specified. + +Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the +right thing to use is bio_endio(bio) instead. + +If the driver is dropping the io_request_lock from its request_fn strategy, +then it just needs to replace that with q->queue_lock instead. + +As described in Sec 1.1, drivers can set max sector size, max segment size +etc per queue now. Drivers that used to define their own merge functions i +to handle things like this can now just use the blk_queue_* functions at +blk_init_queue time. + +Drivers no longer have to map a {partition, sector offset} into the +correct absolute location anymore, this is done by the block layer, so +where a driver received a request ala this before:: + + rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */ + rq->sector = 0; /* first sector on hda5 */ + +it will now see:: + + rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */ + rq->sector = 123128; /* offset from start of disk */ + +As mentioned, there is no virtual mapping of a bio. For DMA, this is +not a problem as the driver probably never will need a virtual mapping. +Instead it needs a bus mapping (dma_map_page for a single segment or +use dma_map_sg for scatter gather) to be able to ship it to the driver. For +PIO drivers (or drivers that need to revert to PIO transfer once in a +while (IDE for example)), where the CPU is doing the actual data +transfer a virtual mapping is needed. If the driver supports highmem I/O, +(Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map +a bio into the virtual address space. + + +8. Prior/Related/Impacted patches +================================= + +8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp) +----------------------------------------------------- + +- orig kiobuf & raw i/o patches (now in 2.4 tree) +- direct kiobuf based i/o to devices (no intermediate bh's) +- page i/o using kiobuf +- kiobuf splitting for lvm (mkp) +- elevator support for kiobuf request merging (axboe) + +8.2. Zero-copy networking (Dave Miller) +--------------------------------------- + +8.3. SGI XFS - pagebuf patches - use of kiobufs +----------------------------------------------- +8.4. Multi-page pioent patch for bio (Christoph Hellwig) +-------------------------------------------------------- +8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11 +-------------------------------------------------------------------- +8.6. Async i/o implementation patch (Ben LaHaise) +------------------------------------------------- +8.7. EVMS layering design (IBM EVMS team) +----------------------------------------- +8.8. Larger page cache size patch (Ben LaHaise) and Large page size (Daniel Phillips) +------------------------------------------------------------------------------------- + + => larger contiguous physical memory buffers + +8.9. VM reservations patch (Ben LaHaise) +---------------------------------------- +8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?) +---------------------------------------------------------- +8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+ +--------------------------------------------------------------------------- +8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, Badari) +------------------------------------------------------------------------------- +8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven) +------------------------------------------------------------------ +8.14 IDE Taskfile i/o patch (Andre Hedrick) +-------------------------------------------- +8.15 Multi-page writeout and readahead patches (Andrew Morton) +--------------------------------------------------------------- +8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy) +----------------------------------------------------------------------- + +9. Other References +=================== + +9.1 The Splice I/O Model +------------------------ + +Larry McVoy (and subsequent discussions on lkml, and Linus' comments - Jan 2001 + +9.2 Discussions about kiobuf and bh design +------------------------------------------ + +On lkml between sct, linus, alan et al - Feb-March 2001 (many of the +initial thoughts that led to bio were brought up in this discussion thread) + +9.3 Discussions on mempool on lkml - Dec 2001. +---------------------------------------------- diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt deleted file mode 100644 index 31c177663ed5..000000000000 --- a/Documentation/block/biodoc.txt +++ /dev/null @@ -1,1076 +0,0 @@ - Notes on the Generic Block Layer Rewrite in Linux 2.5 - ===================================================== - -Notes Written on Jan 15, 2002: - Jens Axboe - Suparna Bhattacharya - -Last Updated May 2, 2002 -September 2003: Updated I/O Scheduler portions - Nick Piggin - -Introduction: - -These are some notes describing some aspects of the 2.5 block layer in the -context of the bio rewrite. The idea is to bring out some of the key -changes and a glimpse of the rationale behind those changes. - -Please mail corrections & suggestions to suparna@in.ibm.com. - -Credits: ---------- - -2.5 bio rewrite: - Jens Axboe - -Many aspects of the generic block layer redesign were driven by and evolved -over discussions, prior patches and the collective experience of several -people. See sections 8 and 9 for a list of some related references. - -The following people helped with review comments and inputs for this -document: - Christoph Hellwig - Arjan van de Ven - Randy Dunlap - Andre Hedrick - -The following people helped with fixes/contributions to the bio patches -while it was still work-in-progress: - David S. Miller - - -Description of Contents: ------------------------- - -1. Scope for tuning of logic to various needs - 1.1 Tuning based on device or low level driver capabilities - - Per-queue parameters - - Highmem I/O support - - I/O scheduler modularization - 1.2 Tuning based on high level requirements/capabilities - 1.2.1 Request Priority/Latency - 1.3 Direct access/bypass to lower layers for diagnostics and special - device operations - 1.3.1 Pre-built commands -2. New flexible and generic but minimalist i/o structure or descriptor - (instead of using buffer heads at the i/o layer) - 2.1 Requirements/Goals addressed - 2.2 The bio struct in detail (multi-page io unit) - 2.3 Changes in the request structure -3. Using bios - 3.1 Setup/teardown (allocation, splitting) - 3.2 Generic bio helper routines - 3.2.1 Traversing segments and completion units in a request - 3.2.2 Setting up DMA scatterlists - 3.2.3 I/O completion - 3.2.4 Implications for drivers that do not interpret bios (don't handle - multiple segments) - 3.3 I/O submission -4. The I/O scheduler -5. Scalability related changes - 5.1 Granular locking: Removal of io_request_lock - 5.2 Prepare for transition to 64 bit sector_t -6. Other Changes/Implications - 6.1 Partition re-mapping handled by the generic block layer -7. A few tips on migration of older drivers -8. A list of prior/related/impacted patches/ideas -9. Other References/Discussion Threads - ---------------------------------------------------------------------------- - -Bio Notes --------- - -Let us discuss the changes in the context of how some overall goals for the -block layer are addressed. - -1. Scope for tuning the generic logic to satisfy various requirements - -The block layer design supports adaptable abstractions to handle common -processing with the ability to tune the logic to an appropriate extent -depending on the nature of the device and the requirements of the caller. -One of the objectives of the rewrite was to increase the degree of tunability -and to enable higher level code to utilize underlying device/driver -capabilities to the maximum extent for better i/o performance. This is -important especially in the light of ever improving hardware capabilities -and application/middleware software designed to take advantage of these -capabilities. - -1.1 Tuning based on low level device / driver capabilities - -Sophisticated devices with large built-in caches, intelligent i/o scheduling -optimizations, high memory DMA support, etc may find some of the -generic processing an overhead, while for less capable devices the -generic functionality is essential for performance or correctness reasons. -Knowledge of some of the capabilities or parameters of the device should be -used at the generic block layer to take the right decisions on -behalf of the driver. - -How is this achieved ? - -Tuning at a per-queue level: - -i. Per-queue limits/values exported to the generic layer by the driver - -Various parameters that the generic i/o scheduler logic uses are set at -a per-queue level (e.g maximum request size, maximum number of segments in -a scatter-gather list, logical block size) - -Some parameters that were earlier available as global arrays indexed by -major/minor are now directly associated with the queue. Some of these may -move into the block device structure in the future. Some characteristics -have been incorporated into a queue flags field rather than separate fields -in themselves. There are blk_queue_xxx functions to set the parameters, -rather than update the fields directly - -Some new queue property settings: - - blk_queue_bounce_limit(q, u64 dma_address) - Enable I/O to highmem pages, dma_address being the - limit. No highmem default. - - blk_queue_max_sectors(q, max_sectors) - Sets two variables that limit the size of the request. - - - The request queue's max_sectors, which is a soft size in - units of 512 byte sectors, and could be dynamically varied - by the core kernel. - - - The request queue's max_hw_sectors, which is a hard limit - and reflects the maximum size request a driver can handle - in units of 512 byte sectors. - - The default for both max_sectors and max_hw_sectors is - 255. The upper limit of max_sectors is 1024. - - blk_queue_max_phys_segments(q, max_segments) - Maximum physical segments you can handle in a request. 128 - default (driver limit). (See 3.2.2) - - blk_queue_max_hw_segments(q, max_segments) - Maximum dma segments the hardware can handle in a request. 128 - default (host adapter limit, after dma remapping). - (See 3.2.2) - - blk_queue_max_segment_size(q, max_seg_size) - Maximum size of a clustered segment, 64kB default. - - blk_queue_logical_block_size(q, logical_block_size) - Lowest possible sector size that the hardware can operate - on, 512 bytes default. - -New queue flags: - - QUEUE_FLAG_CLUSTER (see 3.2.2) - QUEUE_FLAG_QUEUED (see 3.2.4) - - -ii. High-mem i/o capabilities are now considered the default - -The generic bounce buffer logic, present in 2.4, where the block layer would -by default copyin/out i/o requests on high-memory buffers to low-memory buffers -assuming that the driver wouldn't be able to handle it directly, has been -changed in 2.5. The bounce logic is now applied only for memory ranges -for which the device cannot handle i/o. A driver can specify this by -setting the queue bounce limit for the request queue for the device -(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out -where a device is capable of handling high memory i/o. - -In order to enable high-memory i/o where the device is capable of supporting -it, the pci dma mapping routines and associated data structures have now been -modified to accomplish a direct page -> bus translation, without requiring -a virtual address mapping (unlike the earlier scheme of virtual address --> bus translation). So this works uniformly for high-memory pages (which -do not have a corresponding kernel virtual address space mapping) and -low-memory pages. - -Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion -on PCI high mem DMA aspects and mapping of scatter gather lists, and support -for 64 bit PCI. - -Special handling is required only for cases where i/o needs to happen on -pages at physical memory addresses beyond what the device can support. In these -cases, a bounce bio representing a buffer from the supported memory range -is used for performing the i/o with copyin/copyout as needed depending on -the type of the operation. For example, in case of a read operation, the -data read has to be copied to the original buffer on i/o completion, so a -callback routine is set up to do this, while for write, the data is copied -from the original buffer to the bounce buffer prior to issuing the -operation. Since an original buffer may be in a high memory area that's not -mapped in kernel virtual addr, a kmap operation may be required for -performing the copy, and special care may be needed in the completion path -as it may not be in irq context. Special care is also required (by way of -GFP flags) when allocating bounce buffers, to avoid certain highmem -deadlock possibilities. - -It is also possible that a bounce buffer may be allocated from high-memory -area that's not mapped in kernel virtual addr, but within the range that the -device can use directly; so the bounce page may need to be kmapped during -copy operations. [Note: This does not hold in the current implementation, -though] - -There are some situations when pages from high memory may need to -be kmapped, even if bounce buffers are not necessary. For example a device -may need to abort DMA operations and revert to PIO for the transfer, in -which case a virtual mapping of the page is required. For SCSI it is also -done in some scenarios where the low level driver cannot be trusted to -handle a single sg entry correctly. The driver is expected to perform the -kmaps as needed on such occasions as appropriate. A driver could also use -the blk_queue_bounce() routine on its own to bounce highmem i/o to low -memory for specific requests if so desired. - -iii. The i/o scheduler algorithm itself can be replaced/set as appropriate - -As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular -queue or pick from (copy) existing generic schedulers and replace/override -certain portions of it. The 2.5 rewrite provides improved modularization -of the i/o scheduler. There are more pluggable callbacks, e.g for init, -add request, extract request, which makes it possible to abstract specific -i/o scheduling algorithm aspects and details outside of the generic loop. -It also makes it possible to completely hide the implementation details of -the i/o scheduler from block drivers. - -I/O scheduler wrappers are to be used instead of accessing the queue directly. -See section 4. The I/O scheduler for details. - -1.2 Tuning Based on High level code capabilities - -i. Application capabilities for raw i/o - -This comes from some of the high-performance database/middleware -requirements where an application prefers to make its own i/o scheduling -decisions based on an understanding of the access patterns and i/o -characteristics - -ii. High performance filesystems or other higher level kernel code's -capabilities - -Kernel components like filesystems could also take their own i/o scheduling -decisions for optimizing performance. Journalling filesystems may need -some control over i/o ordering. - -What kind of support exists at the generic block layer for this ? - -The flags and rw fields in the bio structure can be used for some tuning -from above e.g indicating that an i/o is just a readahead request, or priority -settings (currently unused). As far as user applications are concerned they -would need an additional mechanism either via open flags or ioctls, or some -other upper level mechanism to communicate such settings to block. - -1.2.1 Request Priority/Latency - -Todo/Under discussion: -Arjan's proposed request priority scheme allows higher levels some broad - control (high/med/low) over the priority of an i/o request vs other pending - requests in the queue. For example it allows reads for bringing in an - executable page on demand to be given a higher priority over pending write - requests which haven't aged too much on the queue. Potentially this priority - could even be exposed to applications in some manner, providing higher level - tunability. Time based aging avoids starvation of lower priority - requests. Some bits in the bi_opf flags field in the bio structure are - intended to be used for this priority information. - - -1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode) - (e.g Diagnostics, Systems Management) - -There are situations where high-level code needs to have direct access to -the low level device capabilities or requires the ability to issue commands -to the device bypassing some of the intermediate i/o layers. -These could, for example, be special control commands issued through ioctl -interfaces, or could be raw read/write commands that stress the drive's -capabilities for certain kinds of fitness tests. Having direct interfaces at -multiple levels without having to pass through upper layers makes -it possible to perform bottom up validation of the i/o path, layer by -layer, starting from the media. - -The normal i/o submission interfaces, e.g submit_bio, could be bypassed -for specially crafted requests which such ioctl or diagnostics -interfaces would typically use, and the elevator add_request routine -can instead be used to directly insert such requests in the queue or preferably -the blk_do_rq routine can be used to place the request on the queue and -wait for completion. Alternatively, sometimes the caller might just -invoke a lower level driver specific interface with the request as a -parameter. - -If the request is a means for passing on special information associated with -the command, then such information is associated with the request->special -field (rather than misuse the request->buffer field which is meant for the -request data buffer's virtual mapping). - -For passing request data, the caller must build up a bio descriptor -representing the concerned memory buffer if the underlying driver interprets -bio segments or uses the block layer end*request* functions for i/o -completion. Alternatively one could directly use the request->buffer field to -specify the virtual address of the buffer, if the driver expects buffer -addresses passed in this way and ignores bio entries for the request type -involved. In the latter case, the driver would modify and manage the -request->buffer, request->sector and request->nr_sectors or -request->current_nr_sectors fields itself rather than using the block layer -end_request or end_that_request_first completion interfaces. -(See 2.3 or Documentation/block/request.txt for a brief explanation of -the request structure fields) - -[TBD: end_that_request_last should be usable even in this case; -Perhaps an end_that_direct_request_first routine could be implemented to make -handling direct requests easier for such drivers; Also for drivers that -expect bios, a helper function could be provided for setting up a bio -corresponding to a data buffer] - - - - -1.3.1 Pre-built Commands - -A request can be created with a pre-built custom command to be sent directly -to the device. The cmd block in the request structure has room for filling -in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for -command pre-building, and the type of the request is now indicated -through rq->flags instead of via rq->cmd) - -The request structure flags can be set up to indicate the type of request -in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC: -packet command issued via blk_do_rq, REQ_SPECIAL: special request). - -It can help to pre-build device commands for requests in advance. -Drivers can now specify a request prepare function (q->prep_rq_fn) that the -block layer would invoke to pre-build device commands for a given request, -or perform other preparatory processing for the request. This is routine is -called by elv_next_request(), i.e. typically just before servicing a request. -(The prepare function would not be called for requests that have RQF_DONTPREP -enabled) - -Aside: - Pre-building could possibly even be done early, i.e before placing the - request on the queue, rather than construct the command on the fly in the - driver while servicing the request queue when it may affect latencies in - interrupt context or responsiveness in general. One way to add early - pre-building would be to do it whenever we fail to merge on a request. - Now REQ_NOMERGE is set in the request flags to skip this one in the future, - which means that it will not change before we feed it to the device. So - the pre-builder hook can be invoked there. - - -2. Flexible and generic but minimalist i/o structure/descriptor. - -2.1 Reason for a new structure and requirements addressed - -Prior to 2.5, buffer heads were used as the unit of i/o at the generic block -layer, and the low level request structure was associated with a chain of -buffer heads for a contiguous i/o request. This led to certain inefficiencies -when it came to large i/o requests and readv/writev style operations, as it -forced such requests to be broken up into small chunks before being passed -on to the generic block layer, only to be merged by the i/o scheduler -when the underlying device was capable of handling the i/o in one shot. -Also, using the buffer head as an i/o structure for i/os that didn't originate -from the buffer cache unnecessarily added to the weight of the descriptors -which were generated for each such chunk. - -The following were some of the goals and expectations considered in the -redesign of the block i/o data structure in 2.5. - -i. Should be appropriate as a descriptor for both raw and buffered i/o - - avoid cache related fields which are irrelevant in the direct/page i/o path, - or filesystem block size alignment restrictions which may not be relevant - for raw i/o. -ii. Ability to represent high-memory buffers (which do not have a virtual - address mapping in kernel address space). -iii.Ability to represent large i/os w/o unnecessarily breaking them up (i.e - greater than PAGE_SIZE chunks in one shot) -iv. At the same time, ability to retain independent identity of i/os from - different sources or i/o units requiring individual completion (e.g. for - latency reasons) -v. Ability to represent an i/o involving multiple physical memory segments - (including non-page aligned page fragments, as specified via readv/writev) - without unnecessarily breaking it up, if the underlying device is capable of - handling it. -vi. Preferably should be based on a memory descriptor structure that can be - passed around different types of subsystems or layers, maybe even - networking, without duplication or extra copies of data/descriptor fields - themselves in the process -vii.Ability to handle the possibility of splits/merges as the structure passes - through layered drivers (lvm, md, evms), with minimal overhead. - -The solution was to define a new structure (bio) for the block layer, -instead of using the buffer head structure (bh) directly, the idea being -avoidance of some associated baggage and limitations. The bio structure -is uniformly used for all i/o at the block layer ; it forms a part of the -bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are -mapped to bio structures. - -2.2 The bio struct - -The bio structure uses a vector representation pointing to an array of tuples -of to describe the i/o buffer, and has various other -fields describing i/o parameters and state that needs to be maintained for -performing the i/o. - -Notice that this representation means that a bio has no virtual address -mapping at all (unlike buffer heads). - -struct bio_vec { - struct page *bv_page; - unsigned short bv_len; - unsigned short bv_offset; -}; - -/* - * main unit of I/O for the block layer and lower layers (ie drivers) - */ -struct bio { - struct bio *bi_next; /* request queue link */ - struct block_device *bi_bdev; /* target device */ - unsigned long bi_flags; /* status, command, etc */ - unsigned long bi_opf; /* low bits: r/w, high: priority */ - - unsigned int bi_vcnt; /* how may bio_vec's */ - struct bvec_iter bi_iter; /* current index into bio_vec array */ - - unsigned int bi_size; /* total size in bytes */ - unsigned short bi_hw_segments; /* segments after DMA remapping */ - unsigned int bi_max; /* max bio_vecs we can hold - used as index into pool */ - struct bio_vec *bi_io_vec; /* the actual vec list */ - bio_end_io_t *bi_end_io; /* bi_end_io (bio) */ - atomic_t bi_cnt; /* pin count: free when it hits zero */ - void *bi_private; -}; - -With this multipage bio design: - -- Large i/os can be sent down in one go using a bio_vec list consisting - of an array of fragments (similar to the way fragments - are represented in the zero-copy network code) -- Splitting of an i/o request across multiple devices (as in the case of - lvm or raid) is achieved by cloning the bio (where the clone points to - the same bi_io_vec array, but with the index and size accordingly modified) -- A linked list of bios is used as before for unrelated merges (*) - this - avoids reallocs and makes independent completions easier to handle. -- Code that traverses the req list can find all the segments of a bio - by using rq_for_each_segment. This handles the fact that a request - has multiple bios, each of which can have multiple segments. -- Drivers which can't process a large bio in one shot can use the bi_iter - field to keep track of the next bio_vec entry to process. - (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE) - [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying - bi_offset an len fields] - -(*) unrelated merges -- a request ends up containing two or more bios that - didn't originate from the same place. - -bi_end_io() i/o callback gets called on i/o completion of the entire bio. - -At a lower level, drivers build a scatter gather list from the merged bios. -The scatter gather list is in the form of an array of -entries with their corresponding dma address mappings filled in at the -appropriate time. As an optimization, contiguous physical pages can be -covered by a single entry where refers to the first page and -covers the range of pages (up to 16 contiguous pages could be covered this -way). There is a helper routine (blk_rq_map_sg) which drivers can use to build -the sg list. - -Note: Right now the only user of bios with more than one page is ll_rw_kio, -which in turn means that only raw I/O uses it (direct i/o may not work -right now). The intent however is to enable clustering of pages etc to -become possible. The pagebuf abstraction layer from SGI also uses multi-page -bios, but that is currently not included in the stock development kernels. -The same is true of Andrew Morton's work-in-progress multipage bio writeout -and readahead patches. - -2.3 Changes in the Request Structure - -The request structure is the structure that gets passed down to low level -drivers. The block layer make_request function builds up a request structure, -places it on the queue and invokes the drivers request_fn. The driver makes -use of block layer helper routine elv_next_request to pull the next request -off the queue. Control or diagnostic functions might bypass block and directly -invoke underlying driver entry points passing in a specially constructed -request structure. - -Only some relevant fields (mainly those which changed or may be referred -to in some of the discussion here) are listed below, not necessarily in -the order in which they occur in the structure (see include/linux/blkdev.h) -Refer to Documentation/block/request.txt for details about all the request -structure fields and a quick reference about the layers which are -supposed to use or modify those fields. - -struct request { - struct list_head queuelist; /* Not meant to be directly accessed by - the driver. - Used by q->elv_next_request_fn - rq->queue is gone - */ - . - . - unsigned char cmd[16]; /* prebuilt command data block */ - unsigned long flags; /* also includes earlier rq->cmd settings */ - . - . - sector_t sector; /* this field is now of type sector_t instead of int - preparation for 64 bit sectors */ - . - . - - /* Number of scatter-gather DMA addr+len pairs after - * physical address coalescing is performed. - */ - unsigned short nr_phys_segments; - - /* Number of scatter-gather addr+len pairs after - * physical and DMA remapping hardware coalescing is performed. - * This is the number of scatter-gather entries the driver - * will actually have to deal with after DMA mapping is done. - */ - unsigned short nr_hw_segments; - - /* Various sector counts */ - unsigned long nr_sectors; /* no. of sectors left: driver modifiable */ - unsigned long hard_nr_sectors; /* block internal copy of above */ - unsigned int current_nr_sectors; /* no. of sectors left in the - current segment:driver modifiable */ - unsigned long hard_cur_sectors; /* block internal copy of the above */ - . - . - int tag; /* command tag associated with request */ - void *special; /* same as before */ - char *buffer; /* valid only for low memory buffers up to - current_nr_sectors */ - . - . - struct bio *bio, *biotail; /* bio list instead of bh */ - struct request_list *rl; -} - -See the req_ops and req_flag_bits definitions for an explanation of the various -flags available. Some bits are used by the block layer or i/o scheduler. - -The behaviour of the various sector counts are almost the same as before, -except that since we have multi-segment bios, current_nr_sectors refers -to the numbers of sectors in the current segment being processed which could -be one of the many segments in the current bio (i.e i/o completion unit). -The nr_sectors value refers to the total number of sectors in the whole -request that remain to be transferred (no change). The purpose of the -hard_xxx values is for block to remember these counts every time it hands -over the request to the driver. These values are updated by block on -end_that_request_first, i.e. every time the driver completes a part of the -transfer and invokes block end*request helpers to mark this. The -driver should not modify these values. The block layer sets up the -nr_sectors and current_nr_sectors fields (based on the corresponding -hard_xxx values and the number of bytes transferred) and updates it on -every transfer that invokes end_that_request_first. It does the same for the -buffer, bio, bio->bi_iter fields too. - -The buffer field is just a virtual address mapping of the current segment -of the i/o buffer in cases where the buffer resides in low-memory. For high -memory i/o, this field is not valid and must not be used by drivers. - -Code that sets up its own request structures and passes them down to -a driver needs to be careful about interoperation with the block layer helper -functions which the driver uses. (Section 1.3) - -3. Using bios - -3.1 Setup/Teardown - -There are routines for managing the allocation, and reference counting, and -freeing of bios (bio_alloc, bio_get, bio_put). - -This makes use of Ingo Molnar's mempool implementation, which enables -subsystems like bio to maintain their own reserve memory pools for guaranteed -deadlock-free allocations during extreme VM load. For example, the VM -subsystem makes use of the block layer to writeout dirty pages in order to be -able to free up memory space, a case which needs careful handling. The -allocation logic draws from the preallocated emergency reserve in situations -where it cannot allocate through normal means. If the pool is empty and it -can wait, then it would trigger action that would help free up memory or -replenish the pool (without deadlocking) and wait for availability in the pool. -If it is in IRQ context, and hence not in a position to do this, allocation -could fail if the pool is empty. In general mempool always first tries to -perform allocation without having to wait, even if it means digging into the -pool as long it is not less that 50% full. - -On a free, memory is released to the pool or directly freed depending on -the current availability in the pool. The mempool interface lets the -subsystem specify the routines to be used for normal alloc and free. In the -case of bio, these routines make use of the standard slab allocator. - -The caller of bio_alloc is expected to taken certain steps to avoid -deadlocks, e.g. avoid trying to allocate more memory from the pool while -already holding memory obtained from the pool. -[TBD: This is a potential issue, though a rare possibility - in the bounce bio allocation that happens in the current code, since - it ends up allocating a second bio from the same pool while - holding the original bio ] - -Memory allocated from the pool should be released back within a limited -amount of time (in the case of bio, that would be after the i/o is completed). -This ensures that if part of the pool has been used up, some work (in this -case i/o) must already be in progress and memory would be available when it -is over. If allocating from multiple pools in the same code path, the order -or hierarchy of allocation needs to be consistent, just the way one deals -with multiple locks. - -The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc()) -for a non-clone bio. There are the 6 pools setup for different size biovecs, -so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the -given size from these slabs. - -The bio_get() routine may be used to hold an extra reference on a bio prior -to i/o submission, if the bio fields are likely to be accessed after the -i/o is issued (since the bio may otherwise get freed in case i/o completion -happens in the meantime). - -The bio_clone_fast() routine may be used to duplicate a bio, where the clone -shares the bio_vec_list with the original bio (i.e. both point to the -same bio_vec_list). This would typically be used for splitting i/o requests -in lvm or md. - -3.2 Generic bio helper Routines - -3.2.1 Traversing segments and completion units in a request - -The macro rq_for_each_segment() should be used for traversing the bios -in the request list (drivers should avoid directly trying to do it -themselves). Using these helpers should also make it easier to cope -with block changes in the future. - - struct req_iterator iter; - rq_for_each_segment(bio_vec, rq, iter) - /* bio_vec is now current segment */ - -I/O completion callbacks are per-bio rather than per-segment, so drivers -that traverse bio chains on completion need to keep that in mind. Drivers -which don't make a distinction between segments and completion units would -need to be reorganized to support multi-segment bios. - -3.2.2 Setting up DMA scatterlists - -The blk_rq_map_sg() helper routine would be used for setting up scatter -gather lists from a request, so a driver need not do it on its own. - - nr_segments = blk_rq_map_sg(q, rq, scatterlist); - -The helper routine provides a level of abstraction which makes it easier -to modify the internals of request to scatterlist conversion down the line -without breaking drivers. The blk_rq_map_sg routine takes care of several -things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER -is set) and correct segment accounting to avoid exceeding the limits which -the i/o hardware can handle, based on various queue properties. - -- Prevents a clustered segment from crossing a 4GB mem boundary -- Avoids building segments that would exceed the number of physical - memory segments that the driver can handle (phys_segments) and the - number that the underlying hardware can handle at once, accounting for - DMA remapping (hw_segments) (i.e. IOMMU aware limits). - -Routines which the low level driver can use to set up the segment limits: - -blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of -hw data segments in a request (i.e. the maximum number of address/length -pairs the host adapter can actually hand to the device at once) - -blk_queue_max_phys_segments() : Sets an upper limit on the maximum number -of physical data segments in a request (i.e. the largest sized scatter list -a driver could handle) - -3.2.3 I/O completion - -The existing generic block layer helper routines end_request, -end_that_request_first and end_that_request_last can be used for i/o -completion (and setting things up so the rest of the i/o or the next -request can be kicked of) as before. With the introduction of multi-page -bio support, end_that_request_first requires an additional argument indicating -the number of sectors completed. - -3.2.4 Implications for drivers that do not interpret bios (don't handle - multiple segments) - -Drivers that do not interpret bios e.g those which do not handle multiple -segments and do not support i/o into high memory addresses (require bounce -buffers) and expect only virtually mapped buffers, can access the rq->buffer -field. As before the driver should use current_nr_sectors to determine the -size of remaining data in the current segment (that is the maximum it can -transfer in one go unless it interprets segments), and rely on the block layer -end_request, or end_that_request_first/last to take care of all accounting -and transparent mapping of the next bio segment when a segment boundary -is crossed on completion of a transfer. (The end*request* functions should -be used if only if the request has come down from block/bio path, not for -direct access requests which only specify rq->buffer without a valid rq->bio) - -3.3 I/O Submission - -The routine submit_bio() is used to submit a single io. Higher level i/o -routines make use of this: - -(a) Buffered i/o: -The routine submit_bh() invokes submit_bio() on a bio corresponding to the -bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before. - -(b) Kiobuf i/o (for raw/direct i/o): -The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and -maps the array to one or more multi-page bios, issuing submit_bio() to -perform the i/o on each of these. - -The embedded bh array in the kiobuf structure has been removed and no -preallocation of bios is done for kiobufs. [The intent is to remove the -blocks array as well, but it's currently in there to kludge around direct i/o.] -Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc. - -Todo/Observation: - - A single kiobuf structure is assumed to correspond to a contiguous range - of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec. - So right now it wouldn't work for direct i/o on non-contiguous blocks. - This is to be resolved. The eventual direction is to replace kiobuf - by kvec's. - - Badari Pulavarty has a patch to implement direct i/o correctly using - bio and kvec. - - -(c) Page i/o: -Todo/Under discussion: - - Andrew Morton's multi-page bio patches attempt to issue multi-page - writeouts (and reads) from the page cache, by directly building up - large bios for submission completely bypassing the usage of buffer - heads. This work is still in progress. - - Christoph Hellwig had some code that uses bios for page-io (rather than - bh). This isn't included in bio as yet. Christoph was also working on a - design for representing virtual/real extents as an entity and modifying - some of the address space ops interfaces to utilize this abstraction rather - than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf - abstraction, but intended to be as lightweight as possible). - -(d) Direct access i/o: -Direct access requests that do not contain bios would be submitted differently -as discussed earlier in section 1.3. - -Aside: - - Kvec i/o: - - Ben LaHaise's aio code uses a slightly different structure instead - of kiobufs, called a kvec_cb. This contains an array of - tuples (very much like the networking code), together with a callback function - and data pointer. This is embedded into a brw_cb structure when passed - to brw_kvec_async(). - - Now it should be possible to directly map these kvecs to a bio. Just as while - cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec - array pointer to point to the veclet array in kvecs. - - TBD: In order for this to work, some changes are needed in the way multi-page - bios are handled today. The values of the tuples in such a vector passed in - from higher level code should not be modified by the block layer in the course - of its request processing, since that would make it hard for the higher layer - to continue to use the vector descriptor (kvec) after i/o completes. Instead, - all such transient state should either be maintained in the request structure, - and passed on in some way to the endio completion routine. - - -4. The I/O scheduler -I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch -queue and specific I/O schedulers. Unless stated otherwise, elevator is used -to refer to both parts and I/O scheduler to specific I/O schedulers. - -Block layer implements generic dispatch queue in block/*.c. -The generic dispatch queue is responsible for requeueing, handling non-fs -requests and all other subtleties. - -Specific I/O schedulers are responsible for ordering normal filesystem -requests. They can also choose to delay certain requests to improve -throughput or whatever purpose. As the plural form indicates, there are -multiple I/O schedulers. They can be built as modules but at least one should -be built inside the kernel. Each queue can choose different one and can also -change to another one dynamically. - -A block layer call to the i/o scheduler follows the convention elv_xxx(). This -calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx -and xxx might not match exactly, but use your imagination. If an elevator -doesn't implement a function, the switch does nothing or some minimal house -keeping work. - -4.1. I/O scheduler API - -The functions an elevator may implement are: (* are mandatory) -elevator_merge_fn called to query requests for merge with a bio - -elevator_merge_req_fn called when two requests get merged. the one - which gets merged into the other one will be - never seen by I/O scheduler again. IOW, after - being merged, the request is gone. - -elevator_merged_fn called when a request in the scheduler has been - involved in a merge. It is used in the deadline - scheduler for example, to reposition the request - if its sorting order has changed. - -elevator_allow_merge_fn called whenever the block layer determines - that a bio can be merged into an existing - request safely. The io scheduler may still - want to stop a merge at this point if it - results in some sort of conflict internally, - this hook allows it to do that. Note however - that two *requests* can still be merged at later - time. Currently the io scheduler has no way to - prevent that. It can only learn about the fact - from elevator_merge_req_fn callback. - -elevator_dispatch_fn* fills the dispatch queue with ready requests. - I/O schedulers are free to postpone requests by - not filling the dispatch queue unless @force - is non-zero. Once dispatched, I/O schedulers - are not allowed to manipulate the requests - - they belong to generic dispatch queue. - -elevator_add_req_fn* called to add a new request into the scheduler - -elevator_former_req_fn -elevator_latter_req_fn These return the request before or after the - one specified in disk sort order. Used by the - block layer to find merge possibilities. - -elevator_completed_req_fn called when a request is completed. - -elevator_may_queue_fn returns true if the scheduler wants to allow the - current context to queue a new request even if - it is over the queue limit. This must be used - very carefully!! - -elevator_set_req_fn -elevator_put_req_fn Must be used to allocate and free any elevator - specific storage for a request. - -elevator_activate_req_fn Called when device driver first sees a request. - I/O schedulers can use this callback to - determine when actual execution of a request - starts. -elevator_deactivate_req_fn Called when device driver decides to delay - a request by requeueing it. - -elevator_init_fn* -elevator_exit_fn Allocate and free any elevator specific storage - for a queue. - -4.2 Request flows seen by I/O schedulers -All requests seen by I/O schedulers strictly follow one of the following three -flows. - - set_req_fn -> - - i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn -> - (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn - ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn - iii. [none] - - -> put_req_fn - -4.3 I/O scheduler implementation -The generic i/o scheduler algorithm attempts to sort/merge/batch requests for -optimal disk scan and request servicing performance (based on generic -principles and device capabilities), optimized for: -i. improved throughput -ii. improved latency -iii. better utilization of h/w & CPU time - -Characteristics: - -i. Binary tree -AS and deadline i/o schedulers use red black binary trees for disk position -sorting and searching, and a fifo linked list for time-based searching. This -gives good scalability and good availability of information. Requests are -almost always dispatched in disk sort order, so a cache is kept of the next -request in sort order to prevent binary tree lookups. - -This arrangement is not a generic block layer characteristic however, so -elevators may implement queues as they please. - -ii. Merge hash -AS and deadline use a hash table indexed by the last sector of a request. This -enables merging code to quickly look up "back merge" candidates, even when -multiple I/O streams are being performed at once on one disk. - -"Front merges", a new request being merged at the front of an existing request, -are far less common than "back merges" due to the nature of most I/O patterns. -Front merges are handled by the binary trees in AS and deadline schedulers. - -iii. Plugging the queue to batch requests in anticipation of opportunities for - merge/sort optimizations - -Plugging is an approach that the current i/o scheduling algorithm resorts to so -that it collects up enough requests in the queue to be able to take -advantage of the sorting/merging logic in the elevator. If the -queue is empty when a request comes in, then it plugs the request queue -(sort of like plugging the bath tub of a vessel to get fluid to build up) -till it fills up with a few more requests, before starting to service -the requests. This provides an opportunity to merge/sort the requests before -passing them down to the device. There are various conditions when the queue is -unplugged (to open up the flow again), either through a scheduled task or -could be on demand. For example wait_on_buffer sets the unplugging going -through sync_buffer() running blk_run_address_space(mapping). Or the caller -can do it explicity through blk_unplug(bdev). So in the read case, -the queue gets explicitly unplugged as part of waiting for completion on that -buffer. - -Aside: - This is kind of controversial territory, as it's not clear if plugging is - always the right thing to do. Devices typically have their own queues, - and allowing a big queue to build up in software, while letting the device be - idle for a while may not always make sense. The trick is to handle the fine - balance between when to plug and when to open up. Also now that we have - multi-page bios being queued in one shot, we may not need to wait to merge - a big request from the broken up pieces coming by. - -4.4 I/O contexts -I/O contexts provide a dynamically allocated per process data area. They may -be used in I/O schedulers, and in the block layer (could be used for IO statis, -priorities for example). See *io_context in block/ll_rw_blk.c, and as-iosched.c -for an example of usage in an i/o scheduler. - - -5. Scalability related changes - -5.1 Granular Locking: io_request_lock replaced by a per-queue lock - -The global io_request_lock has been removed as of 2.5, to avoid -the scalability bottleneck it was causing, and has been replaced by more -granular locking. The request queue structure has a pointer to the -lock to be used for that queue. As a result, locking can now be -per-queue, with a provision for sharing a lock across queues if -necessary (e.g the scsi layer sets the queue lock pointers to the -corresponding adapter lock, which results in a per host locking -granularity). The locking semantics are the same, i.e. locking is -still imposed by the block layer, grabbing the lock before -request_fn execution which it means that lots of older drivers -should still be SMP safe. Drivers are free to drop the queue -lock themselves, if required. Drivers that explicitly used the -io_request_lock for serialization need to be modified accordingly. -Usually it's as easy as adding a global lock: - - static DEFINE_SPINLOCK(my_driver_lock); - -and passing the address to that lock to blk_init_queue(). - -5.2 64 bit sector numbers (sector_t prepares for 64 bit support) - -The sector number used in the bio structure has been changed to sector_t, -which could be defined as 64 bit in preparation for 64 bit sector support. - -6. Other Changes/Implications - -6.1 Partition re-mapping handled by the generic block layer - -In 2.5 some of the gendisk/partition related code has been reorganized. -Now the generic block layer performs partition-remapping early and thus -provides drivers with a sector number relative to whole device, rather than -having to take partition number into account in order to arrive at the true -sector number. The routine blk_partition_remap() is invoked by -generic_make_request even before invoking the queue specific make_request_fn, -so the i/o scheduler also gets to operate on whole disk sector numbers. This -should typically not require changes to block drivers, it just never gets -to invoke its own partition sector offset calculations since all bios -sent are offset from the beginning of the device. - - -7. A Few Tips on Migration of older drivers - -Old-style drivers that just use CURRENT and ignores clustered requests, -may not need much change. The generic layer will automatically handle -clustered requests, multi-page bios, etc for the driver. - -For a low performance driver or hardware that is PIO driven or just doesn't -support scatter-gather changes should be minimal too. - -The following are some points to keep in mind when converting old drivers -to bio. - -Drivers should use elv_next_request to pick up requests and are no longer -supposed to handle looping directly over the request list. -(struct request->queue has been removed) - -Now end_that_request_first takes an additional number_of_sectors argument. -It used to handle always just the first buffer_head in a request, now -it will loop and handle as many sectors (on a bio-segment granularity) -as specified. - -Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the -right thing to use is bio_endio(bio) instead. - -If the driver is dropping the io_request_lock from its request_fn strategy, -then it just needs to replace that with q->queue_lock instead. - -As described in Sec 1.1, drivers can set max sector size, max segment size -etc per queue now. Drivers that used to define their own merge functions i -to handle things like this can now just use the blk_queue_* functions at -blk_init_queue time. - -Drivers no longer have to map a {partition, sector offset} into the -correct absolute location anymore, this is done by the block layer, so -where a driver received a request ala this before: - - rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */ - rq->sector = 0; /* first sector on hda5 */ - - it will now see - - rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */ - rq->sector = 123128; /* offset from start of disk */ - -As mentioned, there is no virtual mapping of a bio. For DMA, this is -not a problem as the driver probably never will need a virtual mapping. -Instead it needs a bus mapping (dma_map_page for a single segment or -use dma_map_sg for scatter gather) to be able to ship it to the driver. For -PIO drivers (or drivers that need to revert to PIO transfer once in a -while (IDE for example)), where the CPU is doing the actual data -transfer a virtual mapping is needed. If the driver supports highmem I/O, -(Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map -a bio into the virtual address space. - - -8. Prior/Related/Impacted patches - -8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp) -- orig kiobuf & raw i/o patches (now in 2.4 tree) -- direct kiobuf based i/o to devices (no intermediate bh's) -- page i/o using kiobuf -- kiobuf splitting for lvm (mkp) -- elevator support for kiobuf request merging (axboe) -8.2. Zero-copy networking (Dave Miller) -8.3. SGI XFS - pagebuf patches - use of kiobufs -8.4. Multi-page pioent patch for bio (Christoph Hellwig) -8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11 -8.6. Async i/o implementation patch (Ben LaHaise) -8.7. EVMS layering design (IBM EVMS team) -8.8. Larger page cache size patch (Ben LaHaise) and - Large page size (Daniel Phillips) - => larger contiguous physical memory buffers -8.9. VM reservations patch (Ben LaHaise) -8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?) -8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+ -8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, - Badari) -8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven) -8.14 IDE Taskfile i/o patch (Andre Hedrick) -8.15 Multi-page writeout and readahead patches (Andrew Morton) -8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy) - -9. Other References: - -9.1 The Splice I/O Model - Larry McVoy (and subsequent discussions on lkml, -and Linus' comments - Jan 2001) -9.2 Discussions about kiobuf and bh design on lkml between sct, linus, alan -et al - Feb-March 2001 (many of the initial thoughts that led to bio were -brought up in this discussion thread) -9.3 Discussions on mempool on lkml - Dec 2001. - diff --git a/Documentation/block/biovecs.rst b/Documentation/block/biovecs.rst new file mode 100644 index 000000000000..86fa66c87172 --- /dev/null +++ b/Documentation/block/biovecs.rst @@ -0,0 +1,146 @@ +====================================== +Immutable biovecs and biovec iterators +====================================== + +Kent Overstreet + +As of 3.13, biovecs should never be modified after a bio has been submitted. +Instead, we have a new struct bvec_iter which represents a range of a biovec - +the iterator will be modified as the bio is completed, not the biovec. + +More specifically, old code that needed to partially complete a bio would +update bi_sector and bi_size, and advance bi_idx to the next biovec. If it +ended up partway through a biovec, it would increment bv_offset and decrement +bv_len by the number of bytes completed in that biovec. + +In the new scheme of things, everything that must be mutated in order to +partially complete a bio is segregated into struct bvec_iter: bi_sector, +bi_size and bi_idx have been moved there; and instead of modifying bv_offset +and bv_len, struct bvec_iter has bi_bvec_done, which represents the number of +bytes completed in the current bvec. + +There are a bunch of new helper macros for hiding the gory details - in +particular, presenting the illusion of partially completed biovecs so that +normal code doesn't have to deal with bi_bvec_done. + + * Driver code should no longer refer to biovecs directly; we now have + bio_iovec() and bio_iter_iovec() macros that return literal struct biovecs, + constructed from the raw biovecs but taking into account bi_bvec_done and + bi_size. + + bio_for_each_segment() has been updated to take a bvec_iter argument + instead of an integer (that corresponded to bi_idx); for a lot of code the + conversion just required changing the types of the arguments to + bio_for_each_segment(). + + * Advancing a bvec_iter is done with bio_advance_iter(); bio_advance() is a + wrapper around bio_advance_iter() that operates on bio->bi_iter, and also + advances the bio integrity's iter if present. + + There is a lower level advance function - bvec_iter_advance() - which takes + a pointer to a biovec, not a bio; this is used by the bio integrity code. + +What's all this get us? +======================= + +Having a real iterator, and making biovecs immutable, has a number of +advantages: + + * Before, iterating over bios was very awkward when you weren't processing + exactly one bvec at a time - for example, bio_copy_data() in fs/bio.c, + which copies the contents of one bio into another. Because the biovecs + wouldn't necessarily be the same size, the old code was tricky convoluted - + it had to walk two different bios at the same time, keeping both bi_idx and + and offset into the current biovec for each. + + The new code is much more straightforward - have a look. This sort of + pattern comes up in a lot of places; a lot of drivers were essentially open + coding bvec iterators before, and having common implementation considerably + simplifies a lot of code. + + * Before, any code that might need to use the biovec after the bio had been + completed (perhaps to copy the data somewhere else, or perhaps to resubmit + it somewhere else if there was an error) had to save the entire bvec array + - again, this was being done in a fair number of places. + + * Biovecs can be shared between multiple bios - a bvec iter can represent an + arbitrary range of an existing biovec, both starting and ending midway + through biovecs. This is what enables efficient splitting of arbitrary + bios. Note that this means we _only_ use bi_size to determine when we've + reached the end of a bio, not bi_vcnt - and the bio_iovec() macro takes + bi_size into account when constructing biovecs. + + * Splitting bios is now much simpler. The old bio_split() didn't even work on + bios with more than a single bvec! Now, we can efficiently split arbitrary + size bios - because the new bio can share the old bio's biovec. + + Care must be taken to ensure the biovec isn't freed while the split bio is + still using it, in case the original bio completes first, though. Using + bio_chain() when splitting bios helps with this. + + * Submitting partially completed bios is now perfectly fine - this comes up + occasionally in stacking block drivers and various code (e.g. md and + bcache) had some ugly workarounds for this. + + It used to be the case that submitting a partially completed bio would work + fine to _most_ devices, but since accessing the raw bvec array was the + norm, not all drivers would respect bi_idx and those would break. Now, + since all drivers _must_ go through the bvec iterator - and have been + audited to make sure they are - submitting partially completed bios is + perfectly fine. + +Other implications: +=================== + + * Almost all usage of bi_idx is now incorrect and has been removed; instead, + where previously you would have used bi_idx you'd now use a bvec_iter, + probably passing it to one of the helper macros. + + I.e. instead of using bio_iovec_idx() (or bio->bi_iovec[bio->bi_idx]), you + now use bio_iter_iovec(), which takes a bvec_iter and returns a + literal struct bio_vec - constructed on the fly from the raw biovec but + taking into account bi_bvec_done (and bi_size). + + * bi_vcnt can't be trusted or relied upon by driver code - i.e. anything that + doesn't actually own the bio. The reason is twofold: firstly, it's not + actually needed for iterating over the bio anymore - we only use bi_size. + Secondly, when cloning a bio and reusing (a portion of) the original bio's + biovec, in order to calculate bi_vcnt for the new bio we'd have to iterate + over all the biovecs in the new bio - which is silly as it's not needed. + + So, don't use bi_vcnt anymore. + + * The current interface allows the block layer to split bios as needed, so we + could eliminate a lot of complexity particularly in stacked drivers. Code + that creates bios can then create whatever size bios are convenient, and + more importantly stacked drivers don't have to deal with both their own bio + size limitations and the limitations of the underlying devices. Thus + there's no need to define ->merge_bvec_fn() callbacks for individual block + drivers. + +Usage of helpers: +================= + +* The following helpers whose names have the suffix of `_all` can only be used + on non-BIO_CLONED bio. They are usually used by filesystem code. Drivers + shouldn't use them because the bio may have been split before it reached the + driver. + +:: + + bio_for_each_segment_all() + bio_first_bvec_all() + bio_first_page_all() + bio_last_bvec_all() + +* The following helpers iterate over single-page segment. The passed 'struct + bio_vec' will contain a single-page IO vector during the iteration:: + + bio_for_each_segment() + bio_for_each_segment_all() + +* The following helpers iterate over multi-page bvec. The passed 'struct + bio_vec' will contain a multi-page IO vector during the iteration:: + + bio_for_each_bvec() + rq_for_each_bvec() diff --git a/Documentation/block/biovecs.txt b/Documentation/block/biovecs.txt deleted file mode 100644 index ce6eccaf5df7..000000000000 --- a/Documentation/block/biovecs.txt +++ /dev/null @@ -1,144 +0,0 @@ - -Immutable biovecs and biovec iterators: -======================================= - -Kent Overstreet - -As of 3.13, biovecs should never be modified after a bio has been submitted. -Instead, we have a new struct bvec_iter which represents a range of a biovec - -the iterator will be modified as the bio is completed, not the biovec. - -More specifically, old code that needed to partially complete a bio would -update bi_sector and bi_size, and advance bi_idx to the next biovec. If it -ended up partway through a biovec, it would increment bv_offset and decrement -bv_len by the number of bytes completed in that biovec. - -In the new scheme of things, everything that must be mutated in order to -partially complete a bio is segregated into struct bvec_iter: bi_sector, -bi_size and bi_idx have been moved there; and instead of modifying bv_offset -and bv_len, struct bvec_iter has bi_bvec_done, which represents the number of -bytes completed in the current bvec. - -There are a bunch of new helper macros for hiding the gory details - in -particular, presenting the illusion of partially completed biovecs so that -normal code doesn't have to deal with bi_bvec_done. - - * Driver code should no longer refer to biovecs directly; we now have - bio_iovec() and bio_iter_iovec() macros that return literal struct biovecs, - constructed from the raw biovecs but taking into account bi_bvec_done and - bi_size. - - bio_for_each_segment() has been updated to take a bvec_iter argument - instead of an integer (that corresponded to bi_idx); for a lot of code the - conversion just required changing the types of the arguments to - bio_for_each_segment(). - - * Advancing a bvec_iter is done with bio_advance_iter(); bio_advance() is a - wrapper around bio_advance_iter() that operates on bio->bi_iter, and also - advances the bio integrity's iter if present. - - There is a lower level advance function - bvec_iter_advance() - which takes - a pointer to a biovec, not a bio; this is used by the bio integrity code. - -What's all this get us? -======================= - -Having a real iterator, and making biovecs immutable, has a number of -advantages: - - * Before, iterating over bios was very awkward when you weren't processing - exactly one bvec at a time - for example, bio_copy_data() in fs/bio.c, - which copies the contents of one bio into another. Because the biovecs - wouldn't necessarily be the same size, the old code was tricky convoluted - - it had to walk two different bios at the same time, keeping both bi_idx and - and offset into the current biovec for each. - - The new code is much more straightforward - have a look. This sort of - pattern comes up in a lot of places; a lot of drivers were essentially open - coding bvec iterators before, and having common implementation considerably - simplifies a lot of code. - - * Before, any code that might need to use the biovec after the bio had been - completed (perhaps to copy the data somewhere else, or perhaps to resubmit - it somewhere else if there was an error) had to save the entire bvec array - - again, this was being done in a fair number of places. - - * Biovecs can be shared between multiple bios - a bvec iter can represent an - arbitrary range of an existing biovec, both starting and ending midway - through biovecs. This is what enables efficient splitting of arbitrary - bios. Note that this means we _only_ use bi_size to determine when we've - reached the end of a bio, not bi_vcnt - and the bio_iovec() macro takes - bi_size into account when constructing biovecs. - - * Splitting bios is now much simpler. The old bio_split() didn't even work on - bios with more than a single bvec! Now, we can efficiently split arbitrary - size bios - because the new bio can share the old bio's biovec. - - Care must be taken to ensure the biovec isn't freed while the split bio is - still using it, in case the original bio completes first, though. Using - bio_chain() when splitting bios helps with this. - - * Submitting partially completed bios is now perfectly fine - this comes up - occasionally in stacking block drivers and various code (e.g. md and - bcache) had some ugly workarounds for this. - - It used to be the case that submitting a partially completed bio would work - fine to _most_ devices, but since accessing the raw bvec array was the - norm, not all drivers would respect bi_idx and those would break. Now, - since all drivers _must_ go through the bvec iterator - and have been - audited to make sure they are - submitting partially completed bios is - perfectly fine. - -Other implications: -=================== - - * Almost all usage of bi_idx is now incorrect and has been removed; instead, - where previously you would have used bi_idx you'd now use a bvec_iter, - probably passing it to one of the helper macros. - - I.e. instead of using bio_iovec_idx() (or bio->bi_iovec[bio->bi_idx]), you - now use bio_iter_iovec(), which takes a bvec_iter and returns a - literal struct bio_vec - constructed on the fly from the raw biovec but - taking into account bi_bvec_done (and bi_size). - - * bi_vcnt can't be trusted or relied upon by driver code - i.e. anything that - doesn't actually own the bio. The reason is twofold: firstly, it's not - actually needed for iterating over the bio anymore - we only use bi_size. - Secondly, when cloning a bio and reusing (a portion of) the original bio's - biovec, in order to calculate bi_vcnt for the new bio we'd have to iterate - over all the biovecs in the new bio - which is silly as it's not needed. - - So, don't use bi_vcnt anymore. - - * The current interface allows the block layer to split bios as needed, so we - could eliminate a lot of complexity particularly in stacked drivers. Code - that creates bios can then create whatever size bios are convenient, and - more importantly stacked drivers don't have to deal with both their own bio - size limitations and the limitations of the underlying devices. Thus - there's no need to define ->merge_bvec_fn() callbacks for individual block - drivers. - -Usage of helpers: -================= - -* The following helpers whose names have the suffix of "_all" can only be used -on non-BIO_CLONED bio. They are usually used by filesystem code. Drivers -shouldn't use them because the bio may have been split before it reached the -driver. - - bio_for_each_segment_all() - bio_first_bvec_all() - bio_first_page_all() - bio_last_bvec_all() - -* The following helpers iterate over single-page segment. The passed 'struct -bio_vec' will contain a single-page IO vector during the iteration - - bio_for_each_segment() - bio_for_each_segment_all() - -* The following helpers iterate over multi-page bvec. The passed 'struct -bio_vec' will contain a multi-page IO vector during the iteration - - bio_for_each_bvec() - rq_for_each_bvec() diff --git a/Documentation/block/capability.rst b/Documentation/block/capability.rst new file mode 100644 index 000000000000..2cf258d64bbe --- /dev/null +++ b/Documentation/block/capability.rst @@ -0,0 +1,18 @@ +=============================== +Generic Block Device Capability +=============================== + +This file documents the sysfs file block//capability + +capability is a hex word indicating which capabilities a specific disk +supports. For more information on bits not listed here, see +include/linux/genhd.h + +GENHD_FL_MEDIA_CHANGE_NOTIFY +---------------------------- + +Value: 4 + +When this bit is set, the disk supports Asynchronous Notification +of media change events. These events will be broadcast to user +space via kernel uevent. diff --git a/Documentation/block/capability.txt b/Documentation/block/capability.txt deleted file mode 100644 index 2f1729424ef4..000000000000 --- a/Documentation/block/capability.txt +++ /dev/null @@ -1,15 +0,0 @@ -Generic Block Device Capability -=============================================================================== -This file documents the sysfs file block//capability - -capability is a hex word indicating which capabilities a specific disk -supports. For more information on bits not listed here, see -include/linux/genhd.h - -Capability Value -------------------------------------------------------------------------------- -GENHD_FL_MEDIA_CHANGE_NOTIFY 4 - When this bit is set, the disk supports Asynchronous Notification - of media change events. These events will be broadcast to user - space via kernel uevent. - diff --git a/Documentation/block/cmdline-partition.rst b/Documentation/block/cmdline-partition.rst new file mode 100644 index 000000000000..530bedff548a --- /dev/null +++ b/Documentation/block/cmdline-partition.rst @@ -0,0 +1,53 @@ +============================================== +Embedded device command line partition parsing +============================================== + +The "blkdevparts" command line option adds support for reading the +block device partition table from the kernel command line. + +It is typically used for fixed block (eMMC) embedded devices. +It has no MBR, so saves storage space. Bootloader can be easily accessed +by absolute address of data on the block device. +Users can easily change the partition. + +The format for the command line is just like mtdparts: + +blkdevparts=[;] + := :[,] + := [@](part-name) + + + block device disk name. Embedded device uses fixed block device. + Its disk name is also fixed, such as: mmcblk0, mmcblk1, mmcblk0boot0. + + + partition size, in bytes, such as: 512, 1m, 1G. + size may contain an optional suffix of (upper or lower case): + + K, M, G, T, P, E. + + "-" is used to denote all remaining space. + + + partition start address, in bytes. + offset may contain an optional suffix of (upper or lower case): + + K, M, G, T, P, E. + +(part-name) + partition name. Kernel sends uevent with "PARTNAME". Application can + create a link to block device partition with the name "PARTNAME". + User space application can access partition by partition name. + +Example: + + eMMC disk names are "mmcblk0" and "mmcblk0boot0". + + bootargs:: + + 'blkdevparts=mmcblk0:1G(data0),1G(data1),-;mmcblk0boot0:1m(boot),-(kernel)' + + dmesg:: + + mmcblk0: p1(data0) p2(data1) p3() + mmcblk0boot0: p1(boot) p2(kernel) diff --git a/Documentation/block/cmdline-partition.txt b/Documentation/block/cmdline-partition.txt deleted file mode 100644 index 760a3f7c3ed4..000000000000 --- a/Documentation/block/cmdline-partition.txt +++ /dev/null @@ -1,46 +0,0 @@ -Embedded device command line partition parsing -===================================================================== - -The "blkdevparts" command line option adds support for reading the -block device partition table from the kernel command line. - -It is typically used for fixed block (eMMC) embedded devices. -It has no MBR, so saves storage space. Bootloader can be easily accessed -by absolute address of data on the block device. -Users can easily change the partition. - -The format for the command line is just like mtdparts: - -blkdevparts=[;] - := :[,] - := [@](part-name) - - - block device disk name. Embedded device uses fixed block device. - Its disk name is also fixed, such as: mmcblk0, mmcblk1, mmcblk0boot0. - - - partition size, in bytes, such as: 512, 1m, 1G. - size may contain an optional suffix of (upper or lower case): - K, M, G, T, P, E. - "-" is used to denote all remaining space. - - - partition start address, in bytes. - offset may contain an optional suffix of (upper or lower case): - K, M, G, T, P, E. - -(part-name) - partition name. Kernel sends uevent with "PARTNAME". Application can - create a link to block device partition with the name "PARTNAME". - User space application can access partition by partition name. - -Example: - eMMC disk names are "mmcblk0" and "mmcblk0boot0". - - bootargs: - 'blkdevparts=mmcblk0:1G(data0),1G(data1),-;mmcblk0boot0:1m(boot),-(kernel)' - - dmesg: - mmcblk0: p1(data0) p2(data1) p3() - mmcblk0boot0: p1(boot) p2(kernel) diff --git a/Documentation/block/data-integrity.rst b/Documentation/block/data-integrity.rst new file mode 100644 index 000000000000..4f2452a95c43 --- /dev/null +++ b/Documentation/block/data-integrity.rst @@ -0,0 +1,291 @@ +============== +Data Integrity +============== + +1. Introduction +=============== + +Modern filesystems feature checksumming of data and metadata to +protect against data corruption. However, the detection of the +corruption is done at read time which could potentially be months +after the data was written. At that point the original data that the +application tried to write is most likely lost. + +The solution is to ensure that the disk is actually storing what the +application meant it to. Recent additions to both the SCSI family +protocols (SBC Data Integrity Field, SCC protection proposal) as well +as SATA/T13 (External Path Protection) try to remedy this by adding +support for appending integrity metadata to an I/O. The integrity +metadata (or protection information in SCSI terminology) includes a +checksum for each sector as well as an incrementing counter that +ensures the individual sectors are written in the right order. And +for some protection schemes also that the I/O is written to the right +place on disk. + +Current storage controllers and devices implement various protective +measures, for instance checksumming and scrubbing. But these +technologies are working in their own isolated domains or at best +between adjacent nodes in the I/O path. The interesting thing about +DIF and the other integrity extensions is that the protection format +is well defined and every node in the I/O path can verify the +integrity of the I/O and reject it if corruption is detected. This +allows not only corruption prevention but also isolation of the point +of failure. + +2. The Data Integrity Extensions +================================ + +As written, the protocol extensions only protect the path between +controller and storage device. However, many controllers actually +allow the operating system to interact with the integrity metadata +(IMD). We have been working with several FC/SAS HBA vendors to enable +the protection information to be transferred to and from their +controllers. + +The SCSI Data Integrity Field works by appending 8 bytes of protection +information to each sector. The data + integrity metadata is stored +in 520 byte sectors on disk. Data + IMD are interleaved when +transferred between the controller and target. The T13 proposal is +similar. + +Because it is highly inconvenient for operating systems to deal with +520 (and 4104) byte sectors, we approached several HBA vendors and +encouraged them to allow separation of the data and integrity metadata +scatter-gather lists. + +The controller will interleave the buffers on write and split them on +read. This means that Linux can DMA the data buffers to and from +host memory without changes to the page cache. + +Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs +is somewhat heavy to compute in software. Benchmarks found that +calculating this checksum had a significant impact on system +performance for a number of workloads. Some controllers allow a +lighter-weight checksum to be used when interfacing with the operating +system. Emulex, for instance, supports the TCP/IP checksum instead. +The IP checksum received from the OS is converted to the 16-bit CRC +when writing and vice versa. This allows the integrity metadata to be +generated by Linux or the application at very low cost (comparable to +software RAID5). + +The IP checksum is weaker than the CRC in terms of detecting bit +errors. However, the strength is really in the separation of the data +buffers and the integrity metadata. These two distinct buffers must +match up for an I/O to complete. + +The separation of the data and integrity metadata buffers as well as +the choice in checksums is referred to as the Data Integrity +Extensions. As these extensions are outside the scope of the protocol +bodies (T10, T13), Oracle and its partners are trying to standardize +them within the Storage Networking Industry Association. + +3. Kernel Changes +================= + +The data integrity framework in Linux enables protection information +to be pinned to I/Os and sent to/received from controllers that +support it. + +The advantage to the integrity extensions in SCSI and SATA is that +they enable us to protect the entire path from application to storage +device. However, at the same time this is also the biggest +disadvantage. It means that the protection information must be in a +format that can be understood by the disk. + +Generally Linux/POSIX applications are agnostic to the intricacies of +the storage devices they are accessing. The virtual filesystem switch +and the block layer make things like hardware sector size and +transport protocols completely transparent to the application. + +However, this level of detail is required when preparing the +protection information to send to a disk. Consequently, the very +concept of an end-to-end protection scheme is a layering violation. +It is completely unreasonable for an application to be aware whether +it is accessing a SCSI or SATA disk. + +The data integrity support implemented in Linux attempts to hide this +from the application. As far as the application (and to some extent +the kernel) is concerned, the integrity metadata is opaque information +that's attached to the I/O. + +The current implementation allows the block layer to automatically +generate the protection information for any I/O. Eventually the +intent is to move the integrity metadata calculation to userspace for +user data. Metadata and other I/O that originates within the kernel +will still use the automatic generation interface. + +Some storage devices allow each hardware sector to be tagged with a +16-bit value. The owner of this tag space is the owner of the block +device. I.e. the filesystem in most cases. The filesystem can use +this extra space to tag sectors as they see fit. Because the tag +space is limited, the block interface allows tagging bigger chunks by +way of interleaving. This way, 8*16 bits of information can be +attached to a typical 4KB filesystem block. + +This also means that applications such as fsck and mkfs will need +access to manipulate the tags from user space. A passthrough +interface for this is being worked on. + + +4. Block Layer Implementation Details +===================================== + +4.1 Bio +------- + +The data integrity patches add a new field to struct bio when +CONFIG_BLK_DEV_INTEGRITY is enabled. bio_integrity(bio) returns a +pointer to a struct bip which contains the bio integrity payload. +Essentially a bip is a trimmed down struct bio which holds a bio_vec +containing the integrity metadata and the required housekeeping +information (bvec pool, vector count, etc.) + +A kernel subsystem can enable data integrity protection on a bio by +calling bio_integrity_alloc(bio). This will allocate and attach the +bip to the bio. + +Individual pages containing integrity metadata can subsequently be +attached using bio_integrity_add_page(). + +bio_free() will automatically free the bip. + + +4.2 Block Device +---------------- + +Because the format of the protection data is tied to the physical +disk, each block device has been extended with a block integrity +profile (struct blk_integrity). This optional profile is registered +with the block layer using blk_integrity_register(). + +The profile contains callback functions for generating and verifying +the protection data, as well as getting and setting application tags. +The profile also contains a few constants to aid in completing, +merging and splitting the integrity metadata. + +Layered block devices will need to pick a profile that's appropriate +for all subdevices. blk_integrity_compare() can help with that. DM +and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6 +will require extra work due to the application tag. + + +5.0 Block Layer Integrity API +============================= + +5.1 Normal Filesystem +--------------------- + + The normal filesystem is unaware that the underlying block device + is capable of sending/receiving integrity metadata. The IMD will + be automatically generated by the block layer at submit_bio() time + in case of a WRITE. A READ request will cause the I/O integrity + to be verified upon completion. + + IMD generation and verification can be toggled using the:: + + /sys/block//integrity/write_generate + + and:: + + /sys/block//integrity/read_verify + + flags. + + +5.2 Integrity-Aware Filesystem +------------------------------ + + A filesystem that is integrity-aware can prepare I/Os with IMD + attached. It can also use the application tag space if this is + supported by the block device. + + + `bool bio_integrity_prep(bio);` + + To generate IMD for WRITE and to set up buffers for READ, the + filesystem must call bio_integrity_prep(bio). + + Prior to calling this function, the bio data direction and start + sector must be set, and the bio should have all data pages + added. It is up to the caller to ensure that the bio does not + change while I/O is in progress. + Complete bio with error if prepare failed for some reson. + + +5.3 Passing Existing Integrity Metadata +--------------------------------------- + + Filesystems that either generate their own integrity metadata or + are capable of transferring IMD from user space can use the + following calls: + + + `struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);` + + Allocates the bio integrity payload and hangs it off of the bio. + nr_pages indicate how many pages of protection data need to be + stored in the integrity bio_vec list (similar to bio_alloc()). + + The integrity payload will be freed at bio_free() time. + + + `int bio_integrity_add_page(bio, page, len, offset);` + + Attaches a page containing integrity metadata to an existing + bio. The bio must have an existing bip, + i.e. bio_integrity_alloc() must have been called. For a WRITE, + the integrity metadata in the pages must be in a format + understood by the target device with the notable exception that + the sector numbers will be remapped as the request traverses the + I/O stack. This implies that the pages added using this call + will be modified during I/O! The first reference tag in the + integrity metadata must have a value of bip->bip_sector. + + Pages can be added using bio_integrity_add_page() as long as + there is room in the bip bio_vec array (nr_pages). + + Upon completion of a READ operation, the attached pages will + contain the integrity metadata received from the storage device. + It is up to the receiver to process them and verify data + integrity upon completion. + + +5.4 Registering A Block Device As Capable Of Exchanging Integrity Metadata +-------------------------------------------------------------------------- + + To enable integrity exchange on a block device the gendisk must be + registered as capable: + + `int blk_integrity_register(gendisk, blk_integrity);` + + The blk_integrity struct is a template and should contain the + following:: + + static struct blk_integrity my_profile = { + .name = "STANDARDSBODY-TYPE-VARIANT-CSUM", + .generate_fn = my_generate_fn, + .verify_fn = my_verify_fn, + .tuple_size = sizeof(struct my_tuple_size), + .tag_size = , + }; + + 'name' is a text string which will be visible in sysfs. This is + part of the userland API so chose it carefully and never change + it. The format is standards body-type-variant. + E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC. + + 'generate_fn' generates appropriate integrity metadata (for WRITE). + + 'verify_fn' verifies that the data buffer matches the integrity + metadata. + + 'tuple_size' must be set to match the size of the integrity + metadata per sector. I.e. 8 for DIF and EPP. + + 'tag_size' must be set to identify how many bytes of tag space + are available per hardware sector. For DIF this is either 2 or + 0 depending on the value of the Control Mode Page ATO bit. + +---------------------------------------------------------------------- + +2007-12-24 Martin K. Petersen diff --git a/Documentation/block/data-integrity.txt b/Documentation/block/data-integrity.txt deleted file mode 100644 index 934c44ea0c57..000000000000 --- a/Documentation/block/data-integrity.txt +++ /dev/null @@ -1,281 +0,0 @@ ----------------------------------------------------------------------- -1. INTRODUCTION - -Modern filesystems feature checksumming of data and metadata to -protect against data corruption. However, the detection of the -corruption is done at read time which could potentially be months -after the data was written. At that point the original data that the -application tried to write is most likely lost. - -The solution is to ensure that the disk is actually storing what the -application meant it to. Recent additions to both the SCSI family -protocols (SBC Data Integrity Field, SCC protection proposal) as well -as SATA/T13 (External Path Protection) try to remedy this by adding -support for appending integrity metadata to an I/O. The integrity -metadata (or protection information in SCSI terminology) includes a -checksum for each sector as well as an incrementing counter that -ensures the individual sectors are written in the right order. And -for some protection schemes also that the I/O is written to the right -place on disk. - -Current storage controllers and devices implement various protective -measures, for instance checksumming and scrubbing. But these -technologies are working in their own isolated domains or at best -between adjacent nodes in the I/O path. The interesting thing about -DIF and the other integrity extensions is that the protection format -is well defined and every node in the I/O path can verify the -integrity of the I/O and reject it if corruption is detected. This -allows not only corruption prevention but also isolation of the point -of failure. - ----------------------------------------------------------------------- -2. THE DATA INTEGRITY EXTENSIONS - -As written, the protocol extensions only protect the path between -controller and storage device. However, many controllers actually -allow the operating system to interact with the integrity metadata -(IMD). We have been working with several FC/SAS HBA vendors to enable -the protection information to be transferred to and from their -controllers. - -The SCSI Data Integrity Field works by appending 8 bytes of protection -information to each sector. The data + integrity metadata is stored -in 520 byte sectors on disk. Data + IMD are interleaved when -transferred between the controller and target. The T13 proposal is -similar. - -Because it is highly inconvenient for operating systems to deal with -520 (and 4104) byte sectors, we approached several HBA vendors and -encouraged them to allow separation of the data and integrity metadata -scatter-gather lists. - -The controller will interleave the buffers on write and split them on -read. This means that Linux can DMA the data buffers to and from -host memory without changes to the page cache. - -Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs -is somewhat heavy to compute in software. Benchmarks found that -calculating this checksum had a significant impact on system -performance for a number of workloads. Some controllers allow a -lighter-weight checksum to be used when interfacing with the operating -system. Emulex, for instance, supports the TCP/IP checksum instead. -The IP checksum received from the OS is converted to the 16-bit CRC -when writing and vice versa. This allows the integrity metadata to be -generated by Linux or the application at very low cost (comparable to -software RAID5). - -The IP checksum is weaker than the CRC in terms of detecting bit -errors. However, the strength is really in the separation of the data -buffers and the integrity metadata. These two distinct buffers must -match up for an I/O to complete. - -The separation of the data and integrity metadata buffers as well as -the choice in checksums is referred to as the Data Integrity -Extensions. As these extensions are outside the scope of the protocol -bodies (T10, T13), Oracle and its partners are trying to standardize -them within the Storage Networking Industry Association. - ----------------------------------------------------------------------- -3. KERNEL CHANGES - -The data integrity framework in Linux enables protection information -to be pinned to I/Os and sent to/received from controllers that -support it. - -The advantage to the integrity extensions in SCSI and SATA is that -they enable us to protect the entire path from application to storage -device. However, at the same time this is also the biggest -disadvantage. It means that the protection information must be in a -format that can be understood by the disk. - -Generally Linux/POSIX applications are agnostic to the intricacies of -the storage devices they are accessing. The virtual filesystem switch -and the block layer make things like hardware sector size and -transport protocols completely transparent to the application. - -However, this level of detail is required when preparing the -protection information to send to a disk. Consequently, the very -concept of an end-to-end protection scheme is a layering violation. -It is completely unreasonable for an application to be aware whether -it is accessing a SCSI or SATA disk. - -The data integrity support implemented in Linux attempts to hide this -from the application. As far as the application (and to some extent -the kernel) is concerned, the integrity metadata is opaque information -that's attached to the I/O. - -The current implementation allows the block layer to automatically -generate the protection information for any I/O. Eventually the -intent is to move the integrity metadata calculation to userspace for -user data. Metadata and other I/O that originates within the kernel -will still use the automatic generation interface. - -Some storage devices allow each hardware sector to be tagged with a -16-bit value. The owner of this tag space is the owner of the block -device. I.e. the filesystem in most cases. The filesystem can use -this extra space to tag sectors as they see fit. Because the tag -space is limited, the block interface allows tagging bigger chunks by -way of interleaving. This way, 8*16 bits of information can be -attached to a typical 4KB filesystem block. - -This also means that applications such as fsck and mkfs will need -access to manipulate the tags from user space. A passthrough -interface for this is being worked on. - - ----------------------------------------------------------------------- -4. BLOCK LAYER IMPLEMENTATION DETAILS - -4.1 BIO - -The data integrity patches add a new field to struct bio when -CONFIG_BLK_DEV_INTEGRITY is enabled. bio_integrity(bio) returns a -pointer to a struct bip which contains the bio integrity payload. -Essentially a bip is a trimmed down struct bio which holds a bio_vec -containing the integrity metadata and the required housekeeping -information (bvec pool, vector count, etc.) - -A kernel subsystem can enable data integrity protection on a bio by -calling bio_integrity_alloc(bio). This will allocate and attach the -bip to the bio. - -Individual pages containing integrity metadata can subsequently be -attached using bio_integrity_add_page(). - -bio_free() will automatically free the bip. - - -4.2 BLOCK DEVICE - -Because the format of the protection data is tied to the physical -disk, each block device has been extended with a block integrity -profile (struct blk_integrity). This optional profile is registered -with the block layer using blk_integrity_register(). - -The profile contains callback functions for generating and verifying -the protection data, as well as getting and setting application tags. -The profile also contains a few constants to aid in completing, -merging and splitting the integrity metadata. - -Layered block devices will need to pick a profile that's appropriate -for all subdevices. blk_integrity_compare() can help with that. DM -and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6 -will require extra work due to the application tag. - - ----------------------------------------------------------------------- -5.0 BLOCK LAYER INTEGRITY API - -5.1 NORMAL FILESYSTEM - - The normal filesystem is unaware that the underlying block device - is capable of sending/receiving integrity metadata. The IMD will - be automatically generated by the block layer at submit_bio() time - in case of a WRITE. A READ request will cause the I/O integrity - to be verified upon completion. - - IMD generation and verification can be toggled using the - - /sys/block//integrity/write_generate - - and - - /sys/block//integrity/read_verify - - flags. - - -5.2 INTEGRITY-AWARE FILESYSTEM - - A filesystem that is integrity-aware can prepare I/Os with IMD - attached. It can also use the application tag space if this is - supported by the block device. - - - bool bio_integrity_prep(bio); - - To generate IMD for WRITE and to set up buffers for READ, the - filesystem must call bio_integrity_prep(bio). - - Prior to calling this function, the bio data direction and start - sector must be set, and the bio should have all data pages - added. It is up to the caller to ensure that the bio does not - change while I/O is in progress. - Complete bio with error if prepare failed for some reson. - - -5.3 PASSING EXISTING INTEGRITY METADATA - - Filesystems that either generate their own integrity metadata or - are capable of transferring IMD from user space can use the - following calls: - - - struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages); - - Allocates the bio integrity payload and hangs it off of the bio. - nr_pages indicate how many pages of protection data need to be - stored in the integrity bio_vec list (similar to bio_alloc()). - - The integrity payload will be freed at bio_free() time. - - - int bio_integrity_add_page(bio, page, len, offset); - - Attaches a page containing integrity metadata to an existing - bio. The bio must have an existing bip, - i.e. bio_integrity_alloc() must have been called. For a WRITE, - the integrity metadata in the pages must be in a format - understood by the target device with the notable exception that - the sector numbers will be remapped as the request traverses the - I/O stack. This implies that the pages added using this call - will be modified during I/O! The first reference tag in the - integrity metadata must have a value of bip->bip_sector. - - Pages can be added using bio_integrity_add_page() as long as - there is room in the bip bio_vec array (nr_pages). - - Upon completion of a READ operation, the attached pages will - contain the integrity metadata received from the storage device. - It is up to the receiver to process them and verify data - integrity upon completion. - - -5.4 REGISTERING A BLOCK DEVICE AS CAPABLE OF EXCHANGING INTEGRITY - METADATA - - To enable integrity exchange on a block device the gendisk must be - registered as capable: - - int blk_integrity_register(gendisk, blk_integrity); - - The blk_integrity struct is a template and should contain the - following: - - static struct blk_integrity my_profile = { - .name = "STANDARDSBODY-TYPE-VARIANT-CSUM", - .generate_fn = my_generate_fn, - .verify_fn = my_verify_fn, - .tuple_size = sizeof(struct my_tuple_size), - .tag_size = , - }; - - 'name' is a text string which will be visible in sysfs. This is - part of the userland API so chose it carefully and never change - it. The format is standards body-type-variant. - E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC. - - 'generate_fn' generates appropriate integrity metadata (for WRITE). - - 'verify_fn' verifies that the data buffer matches the integrity - metadata. - - 'tuple_size' must be set to match the size of the integrity - metadata per sector. I.e. 8 for DIF and EPP. - - 'tag_size' must be set to identify how many bytes of tag space - are available per hardware sector. For DIF this is either 2 or - 0 depending on the value of the Control Mode Page ATO bit. - ----------------------------------------------------------------------- -2007-12-24 Martin K. Petersen diff --git a/Documentation/block/deadline-iosched.rst b/Documentation/block/deadline-iosched.rst new file mode 100644 index 000000000000..9f5c5a4c370e --- /dev/null +++ b/Documentation/block/deadline-iosched.rst @@ -0,0 +1,72 @@ +============================== +Deadline IO scheduler tunables +============================== + +This little file attempts to document how the deadline io scheduler works. +In particular, it will clarify the meaning of the exposed tunables that may be +of interest to power users. + +Selecting IO schedulers +----------------------- +Refer to Documentation/block/switching-sched.rst for information on +selecting an io scheduler on a per-device basis. + +------------------------------------------------------------------------------ + +read_expire (in ms) +----------------------- + +The goal of the deadline io scheduler is to attempt to guarantee a start +service time for a request. As we focus mainly on read latencies, this is +tunable. When a read request first enters the io scheduler, it is assigned +a deadline that is the current time + the read_expire value in units of +milliseconds. + + +write_expire (in ms) +----------------------- + +Similar to read_expire mentioned above, but for writes. + + +fifo_batch (number of requests) +------------------------------------ + +Requests are grouped into ``batches`` of a particular data direction (read or +write) which are serviced in increasing sector order. To limit extra seeking, +deadline expiries are only checked between batches. fifo_batch controls the +maximum number of requests per batch. + +This parameter tunes the balance between per-request latency and aggregate +throughput. When low latency is the primary concern, smaller is better (where +a value of 1 yields first-come first-served behaviour). Increasing fifo_batch +generally improves throughput, at the cost of latency variation. + + +writes_starved (number of dispatches) +-------------------------------------- + +When we have to move requests from the io scheduler queue to the block +device dispatch queue, we always give a preference to reads. However, we +don't want to starve writes indefinitely either. So writes_starved controls +how many times we give preference to reads over writes. When that has been +done writes_starved number of times, we dispatch some writes based on the +same criteria as reads. + + +front_merges (bool) +---------------------- + +Sometimes it happens that a request enters the io scheduler that is contiguous +with a request that is already on the queue. Either it fits in the back of that +request, or it fits at the front. That is called either a back merge candidate +or a front merge candidate. Due to the way files are typically laid out, +back merges are much more common than front merges. For some work loads, you +may even know that it is a waste of time to spend any time attempting to +front merge requests. Setting front_merges to 0 disables this functionality. +Front merges may still occur due to the cached last_merge hint, but since +that comes at basically 0 cost we leave that on. We simply disable the +rbtree front sector lookup when the io scheduler merge function is called. + + +Nov 11 2002, Jens Axboe diff --git a/Documentation/block/deadline-iosched.txt b/Documentation/block/deadline-iosched.txt deleted file mode 100644 index 2d82c80322cb..000000000000 --- a/Documentation/block/deadline-iosched.txt +++ /dev/null @@ -1,75 +0,0 @@ -Deadline IO scheduler tunables -============================== - -This little file attempts to document how the deadline io scheduler works. -In particular, it will clarify the meaning of the exposed tunables that may be -of interest to power users. - -Selecting IO schedulers ------------------------ -Refer to Documentation/block/switching-sched.txt for information on -selecting an io scheduler on a per-device basis. - - -******************************************************************************** - - -read_expire (in ms) ------------ - -The goal of the deadline io scheduler is to attempt to guarantee a start -service time for a request. As we focus mainly on read latencies, this is -tunable. When a read request first enters the io scheduler, it is assigned -a deadline that is the current time + the read_expire value in units of -milliseconds. - - -write_expire (in ms) ------------ - -Similar to read_expire mentioned above, but for writes. - - -fifo_batch (number of requests) ----------- - -Requests are grouped into ``batches'' of a particular data direction (read or -write) which are serviced in increasing sector order. To limit extra seeking, -deadline expiries are only checked between batches. fifo_batch controls the -maximum number of requests per batch. - -This parameter tunes the balance between per-request latency and aggregate -throughput. When low latency is the primary concern, smaller is better (where -a value of 1 yields first-come first-served behaviour). Increasing fifo_batch -generally improves throughput, at the cost of latency variation. - - -writes_starved (number of dispatches) --------------- - -When we have to move requests from the io scheduler queue to the block -device dispatch queue, we always give a preference to reads. However, we -don't want to starve writes indefinitely either. So writes_starved controls -how many times we give preference to reads over writes. When that has been -done writes_starved number of times, we dispatch some writes based on the -same criteria as reads. - - -front_merges (bool) ------------- - -Sometimes it happens that a request enters the io scheduler that is contiguous -with a request that is already on the queue. Either it fits in the back of that -request, or it fits at the front. That is called either a back merge candidate -or a front merge candidate. Due to the way files are typically laid out, -back merges are much more common than front merges. For some work loads, you -may even know that it is a waste of time to spend any time attempting to -front merge requests. Setting front_merges to 0 disables this functionality. -Front merges may still occur due to the cached last_merge hint, but since -that comes at basically 0 cost we leave that on. We simply disable the -rbtree front sector lookup when the io scheduler merge function is called. - - -Nov 11 2002, Jens Axboe - - diff --git a/Documentation/block/index.rst b/Documentation/block/index.rst new file mode 100644 index 000000000000..8cd226a0e86e --- /dev/null +++ b/Documentation/block/index.rst @@ -0,0 +1,25 @@ +:orphan: + +===== +Block +===== + +.. toctree:: + :maxdepth: 1 + + bfq-iosched + biodoc + biovecs + capability + cmdline-partition + data-integrity + deadline-iosched + ioprio + kyber-iosched + null_blk + pr + queue-sysfs + request + stat + switching-sched + writeback_cache_control diff --git a/Documentation/block/ioprio.rst b/Documentation/block/ioprio.rst new file mode 100644 index 000000000000..f72b0de65af7 --- /dev/null +++ b/Documentation/block/ioprio.rst @@ -0,0 +1,182 @@ +=================== +Block io priorities +=================== + + +Intro +----- + +With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io +priorities are supported for reads on files. This enables users to io nice +processes or process groups, similar to what has been possible with cpu +scheduling for ages. This document mainly details the current possibilities +with cfq; other io schedulers do not support io priorities thus far. + +Scheduling classes +------------------ + +CFQ implements three generic scheduling classes that determine how io is +served for a process. + +IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given +higher priority than any other in the system, processes from this class are +given first access to the disk every time. Thus it needs to be used with some +care, one io RT process can starve the entire system. Within the RT class, +there are 8 levels of class data that determine exactly how much time this +process needs the disk for on each service. In the future this might change +to be more directly mappable to performance, by passing in a wanted data +rate instead. + +IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default +for any process that hasn't set a specific io priority. The class data +determines how much io bandwidth the process will get, it's directly mappable +to the cpu nice levels just more coarsely implemented. 0 is the highest +BE prio level, 7 is the lowest. The mapping between cpu nice level and io +nice level is determined as: io_nice = (cpu_nice + 20) / 5. + +IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this +level only get io time when no one else needs the disk. The idle class has no +class data, since it doesn't really apply here. + +Tools +----- + +See below for a sample ionice tool. Usage:: + + # ionice -c -n -p + +If pid isn't given, the current process is assumed. IO priority settings +are inherited on fork, so you can use ionice to start the process at a given +level:: + + # ionice -c2 -n0 /bin/ls + +will run ls at the best-effort scheduling class at the highest priority. +For a running process, you can give the pid instead:: + + # ionice -c1 -n2 -p100 + +will change pid 100 to run at the realtime scheduling class, at priority 2. + +ionice.c tool:: + + #include + #include + #include + #include + #include + #include + #include + + extern int sys_ioprio_set(int, int, int); + extern int sys_ioprio_get(int, int); + + #if defined(__i386__) + #define __NR_ioprio_set 289 + #define __NR_ioprio_get 290 + #elif defined(__ppc__) + #define __NR_ioprio_set 273 + #define __NR_ioprio_get 274 + #elif defined(__x86_64__) + #define __NR_ioprio_set 251 + #define __NR_ioprio_get 252 + #elif defined(__ia64__) + #define __NR_ioprio_set 1274 + #define __NR_ioprio_get 1275 + #else + #error "Unsupported arch" + #endif + + static inline int ioprio_set(int which, int who, int ioprio) + { + return syscall(__NR_ioprio_set, which, who, ioprio); + } + + static inline int ioprio_get(int which, int who) + { + return syscall(__NR_ioprio_get, which, who); + } + + enum { + IOPRIO_CLASS_NONE, + IOPRIO_CLASS_RT, + IOPRIO_CLASS_BE, + IOPRIO_CLASS_IDLE, + }; + + enum { + IOPRIO_WHO_PROCESS = 1, + IOPRIO_WHO_PGRP, + IOPRIO_WHO_USER, + }; + + #define IOPRIO_CLASS_SHIFT 13 + + const char *to_prio[] = { "none", "realtime", "best-effort", "idle", }; + + int main(int argc, char *argv[]) + { + int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE; + int c, pid = 0; + + while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) { + switch (c) { + case 'n': + ioprio = strtol(optarg, NULL, 10); + set = 1; + break; + case 'c': + ioprio_class = strtol(optarg, NULL, 10); + set = 1; + break; + case 'p': + pid = strtol(optarg, NULL, 10); + break; + } + } + + switch (ioprio_class) { + case IOPRIO_CLASS_NONE: + ioprio_class = IOPRIO_CLASS_BE; + break; + case IOPRIO_CLASS_RT: + case IOPRIO_CLASS_BE: + break; + case IOPRIO_CLASS_IDLE: + ioprio = 7; + break; + default: + printf("bad prio class %d\n", ioprio_class); + return 1; + } + + if (!set) { + if (!pid && argv[optind]) + pid = strtol(argv[optind], NULL, 10); + + ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid); + + printf("pid=%d, %d\n", pid, ioprio); + + if (ioprio == -1) + perror("ioprio_get"); + else { + ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT; + ioprio = ioprio & 0xff; + printf("%s: prio %d\n", to_prio[ioprio_class], ioprio); + } + } else { + if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) { + perror("ioprio_set"); + return 1; + } + + if (argv[optind]) + execvp(argv[optind], &argv[optind]); + } + + return 0; + } + + +March 11 2005, Jens Axboe diff --git a/Documentation/block/ioprio.txt b/Documentation/block/ioprio.txt deleted file mode 100644 index 8ed8c59380b4..000000000000 --- a/Documentation/block/ioprio.txt +++ /dev/null @@ -1,183 +0,0 @@ -Block io priorities -=================== - - -Intro ------ - -With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io -priorities are supported for reads on files. This enables users to io nice -processes or process groups, similar to what has been possible with cpu -scheduling for ages. This document mainly details the current possibilities -with cfq; other io schedulers do not support io priorities thus far. - -Scheduling classes ------------------- - -CFQ implements three generic scheduling classes that determine how io is -served for a process. - -IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given -higher priority than any other in the system, processes from this class are -given first access to the disk every time. Thus it needs to be used with some -care, one io RT process can starve the entire system. Within the RT class, -there are 8 levels of class data that determine exactly how much time this -process needs the disk for on each service. In the future this might change -to be more directly mappable to performance, by passing in a wanted data -rate instead. - -IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default -for any process that hasn't set a specific io priority. The class data -determines how much io bandwidth the process will get, it's directly mappable -to the cpu nice levels just more coarsely implemented. 0 is the highest -BE prio level, 7 is the lowest. The mapping between cpu nice level and io -nice level is determined as: io_nice = (cpu_nice + 20) / 5. - -IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this -level only get io time when no one else needs the disk. The idle class has no -class data, since it doesn't really apply here. - -Tools ------ - -See below for a sample ionice tool. Usage: - -# ionice -c -n -p - -If pid isn't given, the current process is assumed. IO priority settings -are inherited on fork, so you can use ionice to start the process at a given -level: - -# ionice -c2 -n0 /bin/ls - -will run ls at the best-effort scheduling class at the highest priority. -For a running process, you can give the pid instead: - -# ionice -c1 -n2 -p100 - -will change pid 100 to run at the realtime scheduling class, at priority 2. - ----> snip ionice.c tool <--- - -#include -#include -#include -#include -#include -#include -#include - -extern int sys_ioprio_set(int, int, int); -extern int sys_ioprio_get(int, int); - -#if defined(__i386__) -#define __NR_ioprio_set 289 -#define __NR_ioprio_get 290 -#elif defined(__ppc__) -#define __NR_ioprio_set 273 -#define __NR_ioprio_get 274 -#elif defined(__x86_64__) -#define __NR_ioprio_set 251 -#define __NR_ioprio_get 252 -#elif defined(__ia64__) -#define __NR_ioprio_set 1274 -#define __NR_ioprio_get 1275 -#else -#error "Unsupported arch" -#endif - -static inline int ioprio_set(int which, int who, int ioprio) -{ - return syscall(__NR_ioprio_set, which, who, ioprio); -} - -static inline int ioprio_get(int which, int who) -{ - return syscall(__NR_ioprio_get, which, who); -} - -enum { - IOPRIO_CLASS_NONE, - IOPRIO_CLASS_RT, - IOPRIO_CLASS_BE, - IOPRIO_CLASS_IDLE, -}; - -enum { - IOPRIO_WHO_PROCESS = 1, - IOPRIO_WHO_PGRP, - IOPRIO_WHO_USER, -}; - -#define IOPRIO_CLASS_SHIFT 13 - -const char *to_prio[] = { "none", "realtime", "best-effort", "idle", }; - -int main(int argc, char *argv[]) -{ - int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE; - int c, pid = 0; - - while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) { - switch (c) { - case 'n': - ioprio = strtol(optarg, NULL, 10); - set = 1; - break; - case 'c': - ioprio_class = strtol(optarg, NULL, 10); - set = 1; - break; - case 'p': - pid = strtol(optarg, NULL, 10); - break; - } - } - - switch (ioprio_class) { - case IOPRIO_CLASS_NONE: - ioprio_class = IOPRIO_CLASS_BE; - break; - case IOPRIO_CLASS_RT: - case IOPRIO_CLASS_BE: - break; - case IOPRIO_CLASS_IDLE: - ioprio = 7; - break; - default: - printf("bad prio class %d\n", ioprio_class); - return 1; - } - - if (!set) { - if (!pid && argv[optind]) - pid = strtol(argv[optind], NULL, 10); - - ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid); - - printf("pid=%d, %d\n", pid, ioprio); - - if (ioprio == -1) - perror("ioprio_get"); - else { - ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT; - ioprio = ioprio & 0xff; - printf("%s: prio %d\n", to_prio[ioprio_class], ioprio); - } - } else { - if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) { - perror("ioprio_set"); - return 1; - } - - if (argv[optind]) - execvp(argv[optind], &argv[optind]); - } - - return 0; -} - ----> snip ionice.c tool <--- - - -March 11 2005, Jens Axboe diff --git a/Documentation/block/kyber-iosched.rst b/Documentation/block/kyber-iosched.rst new file mode 100644 index 000000000000..3e164dd0617c --- /dev/null +++ b/Documentation/block/kyber-iosched.rst @@ -0,0 +1,15 @@ +============================ +Kyber I/O scheduler tunables +============================ + +The only two tunables for the Kyber scheduler are the target latencies for +reads and synchronous writes. Kyber will throttle requests in order to meet +these target latencies. + +read_lat_nsec +------------- +Target latency for reads (in nanoseconds). + +write_lat_nsec +-------------- +Target latency for synchronous writes (in nanoseconds). diff --git a/Documentation/block/kyber-iosched.txt b/Documentation/block/kyber-iosched.txt deleted file mode 100644 index e94feacd7edc..000000000000 --- a/Documentation/block/kyber-iosched.txt +++ /dev/null @@ -1,14 +0,0 @@ -Kyber I/O scheduler tunables -=========================== - -The only two tunables for the Kyber scheduler are the target latencies for -reads and synchronous writes. Kyber will throttle requests in order to meet -these target latencies. - -read_lat_nsec -------------- -Target latency for reads (in nanoseconds). - -write_lat_nsec --------------- -Target latency for synchronous writes (in nanoseconds). diff --git a/Documentation/block/null_blk.rst b/Documentation/block/null_blk.rst new file mode 100644 index 000000000000..31451d80783c --- /dev/null +++ b/Documentation/block/null_blk.rst @@ -0,0 +1,126 @@ +======================== +Null block device driver +======================== + +1. Overview +=========== + +The null block device (/dev/nullb*) is used for benchmarking the various +block-layer implementations. It emulates a block device of X gigabytes in size. +The following instances are possible: + + Single-queue block-layer + + - Request-based. + - Single submission queue per device. + - Implements IO scheduling algorithms (CFQ, Deadline, noop). + + Multi-queue block-layer + + - Request-based. + - Configurable submission queues per device. + + No block-layer (Known as bio-based) + + - Bio-based. IO requests are submitted directly to the device driver. + - Directly accepts bio data structure and returns them. + +All of them have a completion queue for each core in the system. + +2. Module parameters applicable for all instances +================================================= + +queue_mode=[0-2]: Default: 2-Multi-queue + Selects which block-layer the module should instantiate with. + + = ============ + 0 Bio-based + 1 Single-queue + 2 Multi-queue + = ============ + +home_node=[0--nr_nodes]: Default: NUMA_NO_NODE + Selects what CPU node the data structures are allocated from. + +gb=[Size in GB]: Default: 250GB + The size of the device reported to the system. + +bs=[Block size (in bytes)]: Default: 512 bytes + The block size reported to the system. + +nr_devices=[Number of devices]: Default: 1 + Number of block devices instantiated. They are instantiated as /dev/nullb0, + etc. + +irqmode=[0-2]: Default: 1-Soft-irq + The completion mode used for completing IOs to the block-layer. + + = =========================================================================== + 0 None. + 1 Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead + when IOs are issued from another CPU node than the home the device is + connected to. + 2 Timer: Waits a specific period (completion_nsec) for each IO before + completion. + = =========================================================================== + +completion_nsec=[ns]: Default: 10,000ns + Combined with irqmode=2 (timer). The time each completion event must wait. + +submit_queues=[1..nr_cpus]: + The number of submission queues attached to the device driver. If unset, it + defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module + parameter is 1. + +hw_queue_depth=[0..qdepth]: Default: 64 + The hardware queue depth of the device. + +III: Multi-queue specific parameters + +use_per_node_hctx=[0/1]: Default: 0 + + = ===================================================================== + 0 The number of submit queues are set to the value of the submit_queues + parameter. + 1 The multi-queue block layer is instantiated with a hardware dispatch + queue for each CPU node in the system. + = ===================================================================== + +no_sched=[0/1]: Default: 0 + + = ====================================== + 0 nullb* use default blk-mq io scheduler + 1 nullb* doesn't use io scheduler + = ====================================== + +blocking=[0/1]: Default: 0 + + = =============================================================== + 0 Register as a non-blocking blk-mq driver device. + 1 Register as a blocking blk-mq driver device, null_blk will set + the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always + needs to block in its ->queue_rq() function. + = =============================================================== + +shared_tags=[0/1]: Default: 0 + + = ================================================================ + 0 Tag set is not shared. + 1 Tag set shared between devices for blk-mq. Only makes sense with + nr_devices > 1, otherwise there's no tag set to share. + = ================================================================ + +zoned=[0/1]: Default: 0 + + = ====================================================================== + 0 Block device is exposed as a random-access block device. + 1 Block device is exposed as a host-managed zoned block device. Requires + CONFIG_BLK_DEV_ZONED. + = ====================================================================== + +zone_size=[MB]: Default: 256 + Per zone size when exposed as a zoned block device. Must be a power of two. + +zone_nr_conv=[nr_conv]: Default: 0 + The number of conventional zones to create when block device is zoned. If + zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1. diff --git a/Documentation/block/null_blk.txt b/Documentation/block/null_blk.txt deleted file mode 100644 index 41f0a3d33bbd..000000000000 --- a/Documentation/block/null_blk.txt +++ /dev/null @@ -1,99 +0,0 @@ -Null block device driver -================================================================================ - -I. Overview - -The null block device (/dev/nullb*) is used for benchmarking the various -block-layer implementations. It emulates a block device of X gigabytes in size. -The following instances are possible: - - Single-queue block-layer - - Request-based. - - Single submission queue per device. - - Implements IO scheduling algorithms (CFQ, Deadline, noop). - Multi-queue block-layer - - Request-based. - - Configurable submission queues per device. - No block-layer (Known as bio-based) - - Bio-based. IO requests are submitted directly to the device driver. - - Directly accepts bio data structure and returns them. - -All of them have a completion queue for each core in the system. - -II. Module parameters applicable for all instances: - -queue_mode=[0-2]: Default: 2-Multi-queue - Selects which block-layer the module should instantiate with. - - 0: Bio-based. - 1: Single-queue. - 2: Multi-queue. - -home_node=[0--nr_nodes]: Default: NUMA_NO_NODE - Selects what CPU node the data structures are allocated from. - -gb=[Size in GB]: Default: 250GB - The size of the device reported to the system. - -bs=[Block size (in bytes)]: Default: 512 bytes - The block size reported to the system. - -nr_devices=[Number of devices]: Default: 1 - Number of block devices instantiated. They are instantiated as /dev/nullb0, - etc. - -irqmode=[0-2]: Default: 1-Soft-irq - The completion mode used for completing IOs to the block-layer. - - 0: None. - 1: Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead - when IOs are issued from another CPU node than the home the device is - connected to. - 2: Timer: Waits a specific period (completion_nsec) for each IO before - completion. - -completion_nsec=[ns]: Default: 10,000ns - Combined with irqmode=2 (timer). The time each completion event must wait. - -submit_queues=[1..nr_cpus]: - The number of submission queues attached to the device driver. If unset, it - defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module - parameter is 1. - -hw_queue_depth=[0..qdepth]: Default: 64 - The hardware queue depth of the device. - -III: Multi-queue specific parameters - -use_per_node_hctx=[0/1]: Default: 0 - 0: The number of submit queues are set to the value of the submit_queues - parameter. - 1: The multi-queue block layer is instantiated with a hardware dispatch - queue for each CPU node in the system. - -no_sched=[0/1]: Default: 0 - 0: nullb* use default blk-mq io scheduler. - 1: nullb* doesn't use io scheduler. - -blocking=[0/1]: Default: 0 - 0: Register as a non-blocking blk-mq driver device. - 1: Register as a blocking blk-mq driver device, null_blk will set - the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always - needs to block in its ->queue_rq() function. - -shared_tags=[0/1]: Default: 0 - 0: Tag set is not shared. - 1: Tag set shared between devices for blk-mq. Only makes sense with - nr_devices > 1, otherwise there's no tag set to share. - -zoned=[0/1]: Default: 0 - 0: Block device is exposed as a random-access block device. - 1: Block device is exposed as a host-managed zoned block device. Requires - CONFIG_BLK_DEV_ZONED. - -zone_size=[MB]: Default: 256 - Per zone size when exposed as a zoned block device. Must be a power of two. - -zone_nr_conv=[nr_conv]: Default: 0 - The number of conventional zones to create when block device is zoned. If - zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1. diff --git a/Documentation/block/pr.rst b/Documentation/block/pr.rst new file mode 100644 index 000000000000..30ea1c2e39eb --- /dev/null +++ b/Documentation/block/pr.rst @@ -0,0 +1,119 @@ +=============================================== +Block layer support for Persistent Reservations +=============================================== + +The Linux kernel supports a user space interface for simplified +Persistent Reservations which map to block devices that support +these (like SCSI). Persistent Reservations allow restricting +access to block devices to specific initiators in a shared storage +setup. + +This document gives a general overview of the support ioctl commands. +For a more detailed reference please refer the the SCSI Primary +Commands standard, specifically the section on Reservations and the +"PERSISTENT RESERVE IN" and "PERSISTENT RESERVE OUT" commands. + +All implementations are expected to ensure the reservations survive +a power loss and cover all connections in a multi path environment. +These behaviors are optional in SPC but will be automatically applied +by Linux. + + +The following types of reservations are supported: +-------------------------------------------------- + + - PR_WRITE_EXCLUSIVE + Only the initiator that owns the reservation can write to the + device. Any initiator can read from the device. + + - PR_EXCLUSIVE_ACCESS + Only the initiator that owns the reservation can access the + device. + + - PR_WRITE_EXCLUSIVE_REG_ONLY + Only initiators with a registered key can write to the device, + Any initiator can read from the device. + + - PR_EXCLUSIVE_ACCESS_REG_ONLY + Only initiators with a registered key can access the device. + + - PR_WRITE_EXCLUSIVE_ALL_REGS + + Only initiators with a registered key can write to the device, + Any initiator can read from the device. + All initiators with a registered key are considered reservation + holders. + Please reference the SPC spec on the meaning of a reservation + holder if you want to use this type. + + - PR_EXCLUSIVE_ACCESS_ALL_REGS + Only initiators with a registered key can access the device. + All initiators with a registered key are considered reservation + holders. + Please reference the SPC spec on the meaning of a reservation + holder if you want to use this type. + + +The following ioctl are supported: +---------------------------------- + +1. IOC_PR_REGISTER +^^^^^^^^^^^^^^^^^^ + +This ioctl command registers a new reservation if the new_key argument +is non-null. If no existing reservation exists old_key must be zero, +if an existing reservation should be replaced old_key must contain +the old reservation key. + +If the new_key argument is 0 it unregisters the existing reservation passed +in old_key. + + +2. IOC_PR_RESERVE +^^^^^^^^^^^^^^^^^ + +This ioctl command reserves the device and thus restricts access for other +devices based on the type argument. The key argument must be the existing +reservation key for the device as acquired by the IOC_PR_REGISTER, +IOC_PR_REGISTER_IGNORE, IOC_PR_PREEMPT or IOC_PR_PREEMPT_ABORT commands. + + +3. IOC_PR_RELEASE +^^^^^^^^^^^^^^^^^ + +This ioctl command releases the reservation specified by key and flags +and thus removes any access restriction implied by it. + + +4. IOC_PR_PREEMPT +^^^^^^^^^^^^^^^^^ + +This ioctl command releases the existing reservation referred to by +old_key and replaces it with a new reservation of type for the +reservation key new_key. + + +5. IOC_PR_PREEMPT_ABORT +^^^^^^^^^^^^^^^^^^^^^^^ + +This ioctl command works like IOC_PR_PREEMPT except that it also aborts +any outstanding command sent over a connection identified by old_key. + +6. IOC_PR_CLEAR +^^^^^^^^^^^^^^^ + +This ioctl command unregisters both key and any other reservation key +registered with the device and drops any existing reservation. + + +Flags +----- + +All the ioctls have a flag field. Currently only one flag is supported: + + - PR_FL_IGNORE_KEY + Ignore the existing reservation key. This is commonly supported for + IOC_PR_REGISTER, and some implementation may support the flag for + IOC_PR_RESERVE. + +For all unknown flags the kernel will return -EOPNOTSUPP. diff --git a/Documentation/block/pr.txt b/Documentation/block/pr.txt deleted file mode 100644 index ac9b8e70e64b..000000000000 --- a/Documentation/block/pr.txt +++ /dev/null @@ -1,119 +0,0 @@ - -Block layer support for Persistent Reservations -=============================================== - -The Linux kernel supports a user space interface for simplified -Persistent Reservations which map to block devices that support -these (like SCSI). Persistent Reservations allow restricting -access to block devices to specific initiators in a shared storage -setup. - -This document gives a general overview of the support ioctl commands. -For a more detailed reference please refer the the SCSI Primary -Commands standard, specifically the section on Reservations and the -"PERSISTENT RESERVE IN" and "PERSISTENT RESERVE OUT" commands. - -All implementations are expected to ensure the reservations survive -a power loss and cover all connections in a multi path environment. -These behaviors are optional in SPC but will be automatically applied -by Linux. - - -The following types of reservations are supported: --------------------------------------------------- - - - PR_WRITE_EXCLUSIVE - - Only the initiator that owns the reservation can write to the - device. Any initiator can read from the device. - - - PR_EXCLUSIVE_ACCESS - - Only the initiator that owns the reservation can access the - device. - - - PR_WRITE_EXCLUSIVE_REG_ONLY - - Only initiators with a registered key can write to the device, - Any initiator can read from the device. - - - PR_EXCLUSIVE_ACCESS_REG_ONLY - - Only initiators with a registered key can access the device. - - - PR_WRITE_EXCLUSIVE_ALL_REGS - - Only initiators with a registered key can write to the device, - Any initiator can read from the device. - All initiators with a registered key are considered reservation - holders. - Please reference the SPC spec on the meaning of a reservation - holder if you want to use this type. - - - PR_EXCLUSIVE_ACCESS_ALL_REGS - - Only initiators with a registered key can access the device. - All initiators with a registered key are considered reservation - holders. - Please reference the SPC spec on the meaning of a reservation - holder if you want to use this type. - - -The following ioctl are supported: ----------------------------------- - -1. IOC_PR_REGISTER - -This ioctl command registers a new reservation if the new_key argument -is non-null. If no existing reservation exists old_key must be zero, -if an existing reservation should be replaced old_key must contain -the old reservation key. - -If the new_key argument is 0 it unregisters the existing reservation passed -in old_key. - - -2. IOC_PR_RESERVE - -This ioctl command reserves the device and thus restricts access for other -devices based on the type argument. The key argument must be the existing -reservation key for the device as acquired by the IOC_PR_REGISTER, -IOC_PR_REGISTER_IGNORE, IOC_PR_PREEMPT or IOC_PR_PREEMPT_ABORT commands. - - -3. IOC_PR_RELEASE - -This ioctl command releases the reservation specified by key and flags -and thus removes any access restriction implied by it. - - -4. IOC_PR_PREEMPT - -This ioctl command releases the existing reservation referred to by -old_key and replaces it with a new reservation of type for the -reservation key new_key. - - -5. IOC_PR_PREEMPT_ABORT - -This ioctl command works like IOC_PR_PREEMPT except that it also aborts -any outstanding command sent over a connection identified by old_key. - -6. IOC_PR_CLEAR - -This ioctl command unregisters both key and any other reservation key -registered with the device and drops any existing reservation. - - -Flags ------ - -All the ioctls have a flag field. Currently only one flag is supported: - - - PR_FL_IGNORE_KEY - - Ignore the existing reservation key. This is commonly supported for - IOC_PR_REGISTER, and some implementation may support the flag for - IOC_PR_RESERVE. - -For all unknown flags the kernel will return -EOPNOTSUPP. diff --git a/Documentation/block/queue-sysfs.rst b/Documentation/block/queue-sysfs.rst new file mode 100644 index 000000000000..6a8513af9201 --- /dev/null +++ b/Documentation/block/queue-sysfs.rst @@ -0,0 +1,254 @@ +================= +Queue sysfs files +================= + +This text file will detail the queue files that are located in the sysfs tree +for each block device. Note that stacked devices typically do not export +any settings, since their queue merely functions are a remapping target. +These files are the ones found in the /sys/block/xxx/queue/ directory. + +Files denoted with a RO postfix are readonly and the RW postfix means +read-write. + +add_random (RW) +--------------- +This file allows to turn off the disk entropy contribution. Default +value of this file is '1'(on). + +chunk_sectors (RO) +------------------ +This has different meaning depending on the type of the block device. +For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors +of the RAID volume stripe segment. For a zoned block device, either host-aware +or host-managed, chunk_sectors indicates the size in 512B sectors of the zones +of the device, with the eventual exception of the last zone of the device which +may be smaller. + +dax (RO) +-------- +This file indicates whether the device supports Direct Access (DAX), +used by CPU-addressable storage to bypass the pagecache. It shows '1' +if true, '0' if not. + +discard_granularity (RO) +------------------------ +This shows the size of internal allocation of the device in bytes, if +reported by the device. A value of '0' means device does not support +the discard functionality. + +discard_max_hw_bytes (RO) +------------------------- +Devices that support discard functionality may have internal limits on +the number of bytes that can be trimmed or unmapped in a single operation. +The discard_max_bytes parameter is set by the device driver to the maximum +number of bytes that can be discarded in a single operation. Discard +requests issued to the device must not exceed this limit. A discard_max_bytes +value of 0 means that the device does not support discard functionality. + +discard_max_bytes (RW) +---------------------- +While discard_max_hw_bytes is the hardware limit for the device, this +setting is the software limit. Some devices exhibit large latencies when +large discards are issued, setting this value lower will make Linux issue +smaller discards and potentially help reduce latencies induced by large +discard operations. + +discard_zeroes_data (RO) +------------------------ +Obsolete. Always zero. + +fua (RO) +-------- +Whether or not the block driver supports the FUA flag for write requests. +FUA stands for Force Unit Access. If the FUA flag is set that means that +write requests must bypass the volatile cache of the storage device. + +hw_sector_size (RO) +------------------- +This is the hardware sector size of the device, in bytes. + +io_poll (RW) +------------ +When read, this file shows whether polling is enabled (1) or disabled +(0). Writing '0' to this file will disable polling for this device. +Writing any non-zero value will enable this feature. + +io_poll_delay (RW) +------------------ +If polling is enabled, this controls what kind of polling will be +performed. It defaults to -1, which is classic polling. In this mode, +the CPU will repeatedly ask for completions without giving up any time. +If set to 0, a hybrid polling mode is used, where the kernel will attempt +to make an educated guess at when the IO will complete. Based on this +guess, the kernel will put the process issuing IO to sleep for an amount +of time, before entering a classic poll loop. This mode might be a +little slower than pure classic polling, but it will be more efficient. +If set to a value larger than 0, the kernel will put the process issuing +IO to sleep for this amount of microseconds before entering classic +polling. + +io_timeout (RW) +--------------- +io_timeout is the request timeout in milliseconds. If a request does not +complete in this time then the block driver timeout handler is invoked. +That timeout handler can decide to retry the request, to fail it or to start +a device recovery strategy. + +iostats (RW) +------------- +This file is used to control (on/off) the iostats accounting of the +disk. + +logical_block_size (RO) +----------------------- +This is the logical block size of the device, in bytes. + +max_discard_segments (RO) +------------------------- +The maximum number of DMA scatter/gather entries in a discard request. + +max_hw_sectors_kb (RO) +---------------------- +This is the maximum number of kilobytes supported in a single data transfer. + +max_integrity_segments (RO) +--------------------------- +Maximum number of elements in a DMA scatter/gather list with integrity +data that will be submitted by the block layer core to the associated +block driver. + +max_sectors_kb (RW) +------------------- +This is the maximum number of kilobytes that the block layer will allow +for a filesystem request. Must be smaller than or equal to the maximum +size allowed by the hardware. + +max_segments (RO) +----------------- +Maximum number of elements in a DMA scatter/gather list that is submitted +to the associated block driver. + +max_segment_size (RO) +--------------------- +Maximum size in bytes of a single element in a DMA scatter/gather list. + +minimum_io_size (RO) +-------------------- +This is the smallest preferred IO size reported by the device. + +nomerges (RW) +------------- +This enables the user to disable the lookup logic involved with IO +merging requests in the block layer. By default (0) all merges are +enabled. When set to 1 only simple one-hit merges will be tried. When +set to 2 no merge algorithms will be tried (including one-hit or more +complex tree/hash lookups). + +nr_requests (RW) +---------------- +This controls how many requests may be allocated in the block layer for +read or write requests. Note that the total allocated number may be twice +this amount, since it applies only to reads or writes (not the accumulated +sum). + +To avoid priority inversion through request starvation, a request +queue maintains a separate request pool per each cgroup when +CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such +per-block-cgroup request pool. IOW, if there are N block cgroups, +each request queue may have up to N request pools, each independently +regulated by nr_requests. + +nr_zones (RO) +------------- +For zoned block devices (zoned attribute indicating "host-managed" or +"host-aware"), this indicates the total number of zones of the device. +This is always 0 for regular block devices. + +optimal_io_size (RO) +-------------------- +This is the optimal IO size reported by the device. + +physical_block_size (RO) +------------------------ +This is the physical block size of device, in bytes. + +read_ahead_kb (RW) +------------------ +Maximum number of kilobytes to read-ahead for filesystems on this block +device. + +rotational (RW) +--------------- +This file is used to stat if the device is of rotational type or +non-rotational type. + +rq_affinity (RW) +---------------- +If this option is '1', the block layer will migrate request completions to the +cpu "group" that originally submitted the request. For some workloads this +provides a significant reduction in CPU cycles due to caching effects. + +For storage configurations that need to maximize distribution of completion +processing setting this option to '2' forces the completion to run on the +requesting cpu (bypassing the "group" aggregation logic). + +scheduler (RW) +-------------- +When read, this file will display the current and available IO schedulers +for this block device. The currently active IO scheduler will be enclosed +in [] brackets. Writing an IO scheduler name to this file will switch +control of this block device to that new IO scheduler. Note that writing +an IO scheduler name to this file will attempt to load that IO scheduler +module, if it isn't already present in the system. + +write_cache (RW) +---------------- +When read, this file will display whether the device has write back +caching enabled or not. It will return "write back" for the former +case, and "write through" for the latter. Writing to this file can +change the kernels view of the device, but it doesn't alter the +device state. This means that it might not be safe to toggle the +setting from "write back" to "write through", since that will also +eliminate cache flushes issued by the kernel. + +write_same_max_bytes (RO) +------------------------- +This is the number of bytes the device can write in a single write-same +command. A value of '0' means write-same is not supported by this +device. + +wbt_lat_usec (RW) +----------------- +If the device is registered for writeback throttling, then this file shows +the target minimum read latency. If this latency is exceeded in a given +window of time (see wb_window_usec), then the writeback throttling will start +scaling back writes. Writing a value of '0' to this file disables the +feature. Writing a value of '-1' to this file resets the value to the +default setting. + +throttle_sample_time (RW) +------------------------- +This is the time window that blk-throttle samples data, in millisecond. +blk-throttle makes decision based on the samplings. Lower time means cgroups +have more smooth throughput, but higher CPU overhead. This exists only when +CONFIG_BLK_DEV_THROTTLING_LOW is enabled. + +write_zeroes_max_bytes (RO) +--------------------------- +For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of +bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES +is not supported. + +zoned (RO) +---------- +This indicates if the device is a zoned block device and the zone model of the +device if it is indeed zoned. The possible values indicated by zoned are +"none" for regular block devices and "host-aware" or "host-managed" for zoned +block devices. The characteristics of host-aware and host-managed zoned block +devices are described in the ZBC (Zoned Block Commands) and ZAC +(Zoned Device ATA Command Set) standards. These standards also define the +"drive-managed" zone model. However, since drive-managed zoned block devices +do not support zone commands, they will be treated as regular block devices +and zoned will report "none". + +Jens Axboe , February 2009 diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt deleted file mode 100644 index b40b5b7cebd9..000000000000 --- a/Documentation/block/queue-sysfs.txt +++ /dev/null @@ -1,253 +0,0 @@ -Queue sysfs files -================= - -This text file will detail the queue files that are located in the sysfs tree -for each block device. Note that stacked devices typically do not export -any settings, since their queue merely functions are a remapping target. -These files are the ones found in the /sys/block/xxx/queue/ directory. - -Files denoted with a RO postfix are readonly and the RW postfix means -read-write. - -add_random (RW) ----------------- -This file allows to turn off the disk entropy contribution. Default -value of this file is '1'(on). - -chunk_sectors (RO) ------------------- -This has different meaning depending on the type of the block device. -For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors -of the RAID volume stripe segment. For a zoned block device, either host-aware -or host-managed, chunk_sectors indicates the size in 512B sectors of the zones -of the device, with the eventual exception of the last zone of the device which -may be smaller. - -dax (RO) --------- -This file indicates whether the device supports Direct Access (DAX), -used by CPU-addressable storage to bypass the pagecache. It shows '1' -if true, '0' if not. - -discard_granularity (RO) ------------------------ -This shows the size of internal allocation of the device in bytes, if -reported by the device. A value of '0' means device does not support -the discard functionality. - -discard_max_hw_bytes (RO) ----------------------- -Devices that support discard functionality may have internal limits on -the number of bytes that can be trimmed or unmapped in a single operation. -The discard_max_bytes parameter is set by the device driver to the maximum -number of bytes that can be discarded in a single operation. Discard -requests issued to the device must not exceed this limit. A discard_max_bytes -value of 0 means that the device does not support discard functionality. - -discard_max_bytes (RW) ----------------------- -While discard_max_hw_bytes is the hardware limit for the device, this -setting is the software limit. Some devices exhibit large latencies when -large discards are issued, setting this value lower will make Linux issue -smaller discards and potentially help reduce latencies induced by large -discard operations. - -discard_zeroes_data (RO) ------------------------- -Obsolete. Always zero. - -fua (RO) --------- -Whether or not the block driver supports the FUA flag for write requests. -FUA stands for Force Unit Access. If the FUA flag is set that means that -write requests must bypass the volatile cache of the storage device. - -hw_sector_size (RO) -------------------- -This is the hardware sector size of the device, in bytes. - -io_poll (RW) ------------- -When read, this file shows whether polling is enabled (1) or disabled -(0). Writing '0' to this file will disable polling for this device. -Writing any non-zero value will enable this feature. - -io_poll_delay (RW) ------------------- -If polling is enabled, this controls what kind of polling will be -performed. It defaults to -1, which is classic polling. In this mode, -the CPU will repeatedly ask for completions without giving up any time. -If set to 0, a hybrid polling mode is used, where the kernel will attempt -to make an educated guess at when the IO will complete. Based on this -guess, the kernel will put the process issuing IO to sleep for an amount -of time, before entering a classic poll loop. This mode might be a -little slower than pure classic polling, but it will be more efficient. -If set to a value larger than 0, the kernel will put the process issuing -IO to sleep for this amount of microseconds before entering classic -polling. - -io_timeout (RW) ---------------- -io_timeout is the request timeout in milliseconds. If a request does not -complete in this time then the block driver timeout handler is invoked. -That timeout handler can decide to retry the request, to fail it or to start -a device recovery strategy. - -iostats (RW) -------------- -This file is used to control (on/off) the iostats accounting of the -disk. - -logical_block_size (RO) ------------------------ -This is the logical block size of the device, in bytes. - -max_discard_segments (RO) -------------------------- -The maximum number of DMA scatter/gather entries in a discard request. - -max_hw_sectors_kb (RO) ----------------------- -This is the maximum number of kilobytes supported in a single data transfer. - -max_integrity_segments (RO) ---------------------------- -Maximum number of elements in a DMA scatter/gather list with integrity -data that will be submitted by the block layer core to the associated -block driver. - -max_sectors_kb (RW) -------------------- -This is the maximum number of kilobytes that the block layer will allow -for a filesystem request. Must be smaller than or equal to the maximum -size allowed by the hardware. - -max_segments (RO) ------------------ -Maximum number of elements in a DMA scatter/gather list that is submitted -to the associated block driver. - -max_segment_size (RO) ---------------------- -Maximum size in bytes of a single element in a DMA scatter/gather list. - -minimum_io_size (RO) --------------------- -This is the smallest preferred IO size reported by the device. - -nomerges (RW) -------------- -This enables the user to disable the lookup logic involved with IO -merging requests in the block layer. By default (0) all merges are -enabled. When set to 1 only simple one-hit merges will be tried. When -set to 2 no merge algorithms will be tried (including one-hit or more -complex tree/hash lookups). - -nr_requests (RW) ----------------- -This controls how many requests may be allocated in the block layer for -read or write requests. Note that the total allocated number may be twice -this amount, since it applies only to reads or writes (not the accumulated -sum). - -To avoid priority inversion through request starvation, a request -queue maintains a separate request pool per each cgroup when -CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such -per-block-cgroup request pool. IOW, if there are N block cgroups, -each request queue may have up to N request pools, each independently -regulated by nr_requests. - -nr_zones (RO) -------------- -For zoned block devices (zoned attribute indicating "host-managed" or -"host-aware"), this indicates the total number of zones of the device. -This is always 0 for regular block devices. - -optimal_io_size (RO) --------------------- -This is the optimal IO size reported by the device. - -physical_block_size (RO) ------------------------- -This is the physical block size of device, in bytes. - -read_ahead_kb (RW) ------------------- -Maximum number of kilobytes to read-ahead for filesystems on this block -device. - -rotational (RW) ---------------- -This file is used to stat if the device is of rotational type or -non-rotational type. - -rq_affinity (RW) ----------------- -If this option is '1', the block layer will migrate request completions to the -cpu "group" that originally submitted the request. For some workloads this -provides a significant reduction in CPU cycles due to caching effects. - -For storage configurations that need to maximize distribution of completion -processing setting this option to '2' forces the completion to run on the -requesting cpu (bypassing the "group" aggregation logic). - -scheduler (RW) --------------- -When read, this file will display the current and available IO schedulers -for this block device. The currently active IO scheduler will be enclosed -in [] brackets. Writing an IO scheduler name to this file will switch -control of this block device to that new IO scheduler. Note that writing -an IO scheduler name to this file will attempt to load that IO scheduler -module, if it isn't already present in the system. - -write_cache (RW) ----------------- -When read, this file will display whether the device has write back -caching enabled or not. It will return "write back" for the former -case, and "write through" for the latter. Writing to this file can -change the kernels view of the device, but it doesn't alter the -device state. This means that it might not be safe to toggle the -setting from "write back" to "write through", since that will also -eliminate cache flushes issued by the kernel. - -write_same_max_bytes (RO) -------------------------- -This is the number of bytes the device can write in a single write-same -command. A value of '0' means write-same is not supported by this -device. - -wbt_lat_usec (RW) ------------------ -If the device is registered for writeback throttling, then this file shows -the target minimum read latency. If this latency is exceeded in a given -window of time (see wb_window_usec), then the writeback throttling will start -scaling back writes. Writing a value of '0' to this file disables the -feature. Writing a value of '-1' to this file resets the value to the -default setting. - -throttle_sample_time (RW) -------------------------- -This is the time window that blk-throttle samples data, in millisecond. -blk-throttle makes decision based on the samplings. Lower time means cgroups -have more smooth throughput, but higher CPU overhead. This exists only when -CONFIG_BLK_DEV_THROTTLING_LOW is enabled. - -write_zeroes_max_bytes (RO) ---------------------------- -For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of -bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES -is not supported. - -zoned (RO) ----------- -This indicates if the device is a zoned block device and the zone model of the -device if it is indeed zoned. The possible values indicated by zoned are -"none" for regular block devices and "host-aware" or "host-managed" for zoned -block devices. The characteristics of host-aware and host-managed zoned block -devices are described in the ZBC (Zoned Block Commands) and ZAC -(Zoned Device ATA Command Set) standards. These standards also define the -"drive-managed" zone model. However, since drive-managed zoned block devices -do not support zone commands, they will be treated as regular block devices -and zoned will report "none". - -Jens Axboe , February 2009 diff --git a/Documentation/block/request.rst b/Documentation/block/request.rst new file mode 100644 index 000000000000..747021e1ffdb --- /dev/null +++ b/Documentation/block/request.rst @@ -0,0 +1,99 @@ +============================ +struct request documentation +============================ + +Jens Axboe 27/05/02 + + +.. FIXME: + No idea about what does mean - seems just some noise, so comment it + + 1.0 + Index + + 2.0 Struct request members classification + + 2.1 struct request members explanation + + 3.0 + + + 2.0 + + + +Short explanation of request members +==================================== + +Classification flags: + + = ==================== + D driver member + B block layer member + I I/O scheduler member + = ==================== + +Unless an entry contains a D classification, a device driver must not access +this member. Some members may contain D classifications, but should only be +access through certain macros or functions (eg ->flags). + + + +=============================== ======= ======================================= +Member Flag Comment +=============================== ======= ======================================= +struct list_head queuelist BI Organization on various internal + queues + +``void *elevator_private`` I I/O scheduler private data + +unsigned char cmd[16] D Driver can use this for setting up + a cdb before execution, see + blk_queue_prep_rq + +unsigned long flags DBI Contains info about data direction, + request type, etc. + +int rq_status D Request status bits + +kdev_t rq_dev DBI Target device + +int errors DB Error counts + +sector_t sector DBI Target location + +unsigned long hard_nr_sectors B Used to keep sector sane + +unsigned long nr_sectors DBI Total number of sectors in request + +unsigned long hard_nr_sectors B Used to keep nr_sectors sane + +unsigned short nr_phys_segments DB Number of physical scatter gather + segments in a request + +unsigned short nr_hw_segments DB Number of hardware scatter gather + segments in a request + +unsigned int current_nr_sectors DB Number of sectors in first segment + of request + +unsigned int hard_cur_sectors B Used to keep current_nr_sectors sane + +int tag DB TCQ tag, if assigned + +``void *special`` D Free to be used by driver + +``char *buffer`` D Map of first segment, also see + section on bouncing SECTION + +``struct completion *waiting`` D Can be used by driver to get signalled + on request completion + +``struct bio *bio`` DBI First bio in request + +``struct bio *biotail`` DBI Last bio in request + +``struct request_queue *q`` DB Request queue this request belongs to + +``struct request_list *rl`` B Request list this request came from +=============================== ======= ======================================= diff --git a/Documentation/block/request.txt b/Documentation/block/request.txt deleted file mode 100644 index 754e104ed369..000000000000 --- a/Documentation/block/request.txt +++ /dev/null @@ -1,88 +0,0 @@ - -struct request documentation - -Jens Axboe 27/05/02 - -1.0 -Index - -2.0 Struct request members classification - - 2.1 struct request members explanation - -3.0 - - -2.0 -Short explanation of request members - -Classification flags: - - D driver member - B block layer member - I I/O scheduler member - -Unless an entry contains a D classification, a device driver must not access -this member. Some members may contain D classifications, but should only be -access through certain macros or functions (eg ->flags). - - - -2.1 -Member Flag Comment ------- ---- ------- - -struct list_head queuelist BI Organization on various internal - queues - -void *elevator_private I I/O scheduler private data - -unsigned char cmd[16] D Driver can use this for setting up - a cdb before execution, see - blk_queue_prep_rq - -unsigned long flags DBI Contains info about data direction, - request type, etc. - -int rq_status D Request status bits - -kdev_t rq_dev DBI Target device - -int errors DB Error counts - -sector_t sector DBI Target location - -unsigned long hard_nr_sectors B Used to keep sector sane - -unsigned long nr_sectors DBI Total number of sectors in request - -unsigned long hard_nr_sectors B Used to keep nr_sectors sane - -unsigned short nr_phys_segments DB Number of physical scatter gather - segments in a request - -unsigned short nr_hw_segments DB Number of hardware scatter gather - segments in a request - -unsigned int current_nr_sectors DB Number of sectors in first segment - of request - -unsigned int hard_cur_sectors B Used to keep current_nr_sectors sane - -int tag DB TCQ tag, if assigned - -void *special D Free to be used by driver - -char *buffer D Map of first segment, also see - section on bouncing SECTION - -struct completion *waiting D Can be used by driver to get signalled - on request completion - -struct bio *bio DBI First bio in request - -struct bio *biotail DBI Last bio in request - -struct request_queue *q DB Request queue this request belongs to - -struct request_list *rl B Request list this request came from diff --git a/Documentation/block/stat.rst b/Documentation/block/stat.rst new file mode 100644 index 000000000000..9c07bc22b0bc --- /dev/null +++ b/Documentation/block/stat.rst @@ -0,0 +1,93 @@ +=============================================== +Block layer statistics in /sys/block//stat +=============================================== + +This file documents the contents of the /sys/block//stat file. + +The stat file provides several statistics about the state of block +device . + +Q. + Why are there multiple statistics in a single file? Doesn't sysfs + normally contain a single value per file? + +A. + By having a single file, the kernel can guarantee that the statistics + represent a consistent snapshot of the state of the device. If the + statistics were exported as multiple files containing one statistic + each, it would be impossible to guarantee that a set of readings + represent a single point in time. + +The stat file consists of a single line of text containing 11 decimal +values separated by whitespace. The fields are summarized in the +following table, and described in more detail below. + + +=============== ============= ================================================= +Name units description +=============== ============= ================================================= +read I/Os requests number of read I/Os processed +read merges requests number of read I/Os merged with in-queue I/O +read sectors sectors number of sectors read +read ticks milliseconds total wait time for read requests +write I/Os requests number of write I/Os processed +write merges requests number of write I/Os merged with in-queue I/O +write sectors sectors number of sectors written +write ticks milliseconds total wait time for write requests +in_flight requests number of I/Os currently in flight +io_ticks milliseconds total time this block device has been active +time_in_queue milliseconds total wait time for all requests +discard I/Os requests number of discard I/Os processed +discard merges requests number of discard I/Os merged with in-queue I/O +discard sectors sectors number of sectors discarded +discard ticks milliseconds total wait time for discard requests +=============== ============= ================================================= + +read I/Os, write I/Os, discard I/0s +=================================== + +These values increment when an I/O request completes. + +read merges, write merges, discard merges +========================================= + +These values increment when an I/O request is merged with an +already-queued I/O request. + +read sectors, write sectors, discard_sectors +============================================ + +These values count the number of sectors read from, written to, or +discarded from this block device. The "sectors" in question are the +standard UNIX 512-byte sectors, not any device- or filesystem-specific +block size. The counters are incremented when the I/O completes. + +read ticks, write ticks, discard ticks +====================================== + +These values count the number of milliseconds that I/O requests have +waited on this block device. If there are multiple I/O requests waiting, +these values will increase at a rate greater than 1000/second; for +example, if 60 read requests wait for an average of 30 ms, the read_ticks +field will increase by 60*30 = 1800. + +in_flight +========= + +This value counts the number of I/O requests that have been issued to +the device driver but have not yet completed. It does not include I/O +requests that are in the queue but not yet issued to the device driver. + +io_ticks +======== + +This value counts the number of milliseconds during which the device has +had I/O requests queued. + +time_in_queue +============= + +This value counts the number of milliseconds that I/O requests have waited +on this block device. If there are multiple I/O requests waiting, this +value will increase as the product of the number of milliseconds times the +number of requests waiting (see "read ticks" above for an example). diff --git a/Documentation/block/stat.txt b/Documentation/block/stat.txt deleted file mode 100644 index 0aace9cc536c..000000000000 --- a/Documentation/block/stat.txt +++ /dev/null @@ -1,86 +0,0 @@ -Block layer statistics in /sys/block//stat -=============================================== - -This file documents the contents of the /sys/block//stat file. - -The stat file provides several statistics about the state of block -device . - -Q. Why are there multiple statistics in a single file? Doesn't sysfs - normally contain a single value per file? -A. By having a single file, the kernel can guarantee that the statistics - represent a consistent snapshot of the state of the device. If the - statistics were exported as multiple files containing one statistic - each, it would be impossible to guarantee that a set of readings - represent a single point in time. - -The stat file consists of a single line of text containing 11 decimal -values separated by whitespace. The fields are summarized in the -following table, and described in more detail below. - -Name units description ----- ----- ----------- -read I/Os requests number of read I/Os processed -read merges requests number of read I/Os merged with in-queue I/O -read sectors sectors number of sectors read -read ticks milliseconds total wait time for read requests -write I/Os requests number of write I/Os processed -write merges requests number of write I/Os merged with in-queue I/O -write sectors sectors number of sectors written -write ticks milliseconds total wait time for write requests -in_flight requests number of I/Os currently in flight -io_ticks milliseconds total time this block device has been active -time_in_queue milliseconds total wait time for all requests -discard I/Os requests number of discard I/Os processed -discard merges requests number of discard I/Os merged with in-queue I/O -discard sectors sectors number of sectors discarded -discard ticks milliseconds total wait time for discard requests - -read I/Os, write I/Os, discard I/0s -=================================== - -These values increment when an I/O request completes. - -read merges, write merges, discard merges -========================================= - -These values increment when an I/O request is merged with an -already-queued I/O request. - -read sectors, write sectors, discard_sectors -============================================ - -These values count the number of sectors read from, written to, or -discarded from this block device. The "sectors" in question are the -standard UNIX 512-byte sectors, not any device- or filesystem-specific -block size. The counters are incremented when the I/O completes. - -read ticks, write ticks, discard ticks -====================================== - -These values count the number of milliseconds that I/O requests have -waited on this block device. If there are multiple I/O requests waiting, -these values will increase at a rate greater than 1000/second; for -example, if 60 read requests wait for an average of 30 ms, the read_ticks -field will increase by 60*30 = 1800. - -in_flight -========= - -This value counts the number of I/O requests that have been issued to -the device driver but have not yet completed. It does not include I/O -requests that are in the queue but not yet issued to the device driver. - -io_ticks -======== - -This value counts the number of milliseconds during which the device has -had I/O requests queued. - -time_in_queue -============= - -This value counts the number of milliseconds that I/O requests have waited -on this block device. If there are multiple I/O requests waiting, this -value will increase as the product of the number of milliseconds times the -number of requests waiting (see "read ticks" above for an example). diff --git a/Documentation/block/switching-sched.rst b/Documentation/block/switching-sched.rst new file mode 100644 index 000000000000..42042417380e --- /dev/null +++ b/Documentation/block/switching-sched.rst @@ -0,0 +1,39 @@ +=================== +Switching Scheduler +=================== + +To choose IO schedulers at boot time, use the argument 'elevator=deadline'. +'noop' and 'cfq' (the default) are also available. IO schedulers are assigned +globally at boot time only presently. + +Each io queue has a set of io scheduler tunables associated with it. These +tunables control how the io scheduler works. You can find these entries +in:: + + /sys/block//queue/iosched + +assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted, +you can do so by typing:: + + # mount none /sys -t sysfs + +It is possible to change the IO scheduler for a given block device on +the fly to select one of mq-deadline, none, bfq, or kyber schedulers - +which can improve that device's throughput. + +To set a specific scheduler, simply do this:: + + echo SCHEDNAME > /sys/block/DEV/queue/scheduler + +where SCHEDNAME is the name of a defined IO scheduler, and DEV is the +device name (hda, hdb, sga, or whatever you happen to have). + +The list of defined schedulers can be found by simply doing +a "cat /sys/block/DEV/queue/scheduler" - the list of valid names +will be displayed, with the currently selected scheduler in brackets:: + + # cat /sys/block/sda/queue/scheduler + [mq-deadline] kyber bfq none + # echo none >/sys/block/sda/queue/scheduler + # cat /sys/block/sda/queue/scheduler + [none] mq-deadline kyber bfq diff --git a/Documentation/block/switching-sched.txt b/Documentation/block/switching-sched.txt deleted file mode 100644 index 7977f6fb8b20..000000000000 --- a/Documentation/block/switching-sched.txt +++ /dev/null @@ -1,35 +0,0 @@ -To choose IO schedulers at boot time, use the argument 'elevator=deadline'. -'noop' and 'cfq' (the default) are also available. IO schedulers are assigned -globally at boot time only presently. - -Each io queue has a set of io scheduler tunables associated with it. These -tunables control how the io scheduler works. You can find these entries -in: - -/sys/block//queue/iosched - -assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted, -you can do so by typing: - -# mount none /sys -t sysfs - -It is possible to change the IO scheduler for a given block device on -the fly to select one of mq-deadline, none, bfq, or kyber schedulers - -which can improve that device's throughput. - -To set a specific scheduler, simply do this: - -echo SCHEDNAME > /sys/block/DEV/queue/scheduler - -where SCHEDNAME is the name of a defined IO scheduler, and DEV is the -device name (hda, hdb, sga, or whatever you happen to have). - -The list of defined schedulers can be found by simply doing -a "cat /sys/block/DEV/queue/scheduler" - the list of valid names -will be displayed, with the currently selected scheduler in brackets: - -# cat /sys/block/sda/queue/scheduler -[mq-deadline] kyber bfq none -# echo none >/sys/block/sda/queue/scheduler -# cat /sys/block/sda/queue/scheduler -[none] mq-deadline kyber bfq diff --git a/Documentation/block/writeback_cache_control.rst b/Documentation/block/writeback_cache_control.rst new file mode 100644 index 000000000000..2c752c57c14c --- /dev/null +++ b/Documentation/block/writeback_cache_control.rst @@ -0,0 +1,86 @@ +========================================== +Explicit volatile write back cache control +========================================== + +Introduction +------------ + +Many storage devices, especially in the consumer market, come with volatile +write back caches. That means the devices signal I/O completion to the +operating system before data actually has hit the non-volatile storage. This +behavior obviously speeds up various workloads, but it means the operating +system needs to force data out to the non-volatile storage when it performs +a data integrity operation like fsync, sync or an unmount. + +The Linux block layer provides two simple mechanisms that let filesystems +control the caching behavior of the storage device. These mechanisms are +a forced cache flush, and the Force Unit Access (FUA) flag for requests. + + +Explicit cache flushes +---------------------- + +The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from +the filesystem and will make sure the volatile cache of the storage device +has been flushed before the actual I/O operation is started. This explicitly +guarantees that previously completed write requests are on non-volatile +storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be +set on an otherwise empty bio structure, which causes only an explicit cache +flush without any dependent I/O. It is recommend to use +the blkdev_issue_flush() helper for a pure cache flush. + + +Forced Unit Access +------------------ + +The REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the +filesystem and will make sure that I/O completion for this request is only +signaled after the data has been committed to non-volatile storage. + + +Implementation details for filesystems +-------------------------------------- + +Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to +worry if the underlying devices need any explicit cache flushing and how +the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags +may both be set on a single bio. + + +Implementation details for make_request_fn based block drivers +-------------------------------------------------------------- + +These drivers will always see the REQ_PREFLUSH and REQ_FUA bits as they sit +directly below the submit_bio interface. For remapping drivers the REQ_FUA +bits need to be propagated to underlying devices, and a global flush needs +to be implemented for bios with the REQ_PREFLUSH bit set. For real device +drivers that do not have a volatile cache the REQ_PREFLUSH and REQ_FUA bits +on non-empty bios can simply be ignored, and REQ_PREFLUSH requests without +data can be completed successfully without doing any work. Drivers for +devices with volatile caches need to implement the support for these +flags themselves without any help from the block layer. + + +Implementation details for request_fn based block drivers +--------------------------------------------------------- + +For devices that do not support volatile write caches there is no driver +support required, the block layer completes empty REQ_PREFLUSH requests before +entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from +requests that have a payload. For devices with volatile write caches the +driver needs to tell the block layer that it supports flushing caches by +doing:: + + blk_queue_write_cache(sdkp->disk->queue, true, false); + +and handle empty REQ_OP_FLUSH requests in its prep_fn/request_fn. Note that +REQ_PREFLUSH requests with a payload are automatically turned into a sequence +of an empty REQ_OP_FLUSH request followed by the actual write by the block +layer. For devices that also support the FUA bit the block layer needs +to be told to pass through the REQ_FUA bit using:: + + blk_queue_write_cache(sdkp->disk->queue, true, true); + +and the driver must handle write requests that have the REQ_FUA bit set +in prep_fn/request_fn. If the FUA bit is not natively supported the block +layer turns it into an empty REQ_OP_FLUSH request after the actual write. diff --git a/Documentation/block/writeback_cache_control.txt b/Documentation/block/writeback_cache_control.txt deleted file mode 100644 index 8a6bdada5f6b..000000000000 --- a/Documentation/block/writeback_cache_control.txt +++ /dev/null @@ -1,86 +0,0 @@ - -Explicit volatile write back cache control -===================================== - -Introduction ------------- - -Many storage devices, especially in the consumer market, come with volatile -write back caches. That means the devices signal I/O completion to the -operating system before data actually has hit the non-volatile storage. This -behavior obviously speeds up various workloads, but it means the operating -system needs to force data out to the non-volatile storage when it performs -a data integrity operation like fsync, sync or an unmount. - -The Linux block layer provides two simple mechanisms that let filesystems -control the caching behavior of the storage device. These mechanisms are -a forced cache flush, and the Force Unit Access (FUA) flag for requests. - - -Explicit cache flushes ----------------------- - -The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from -the filesystem and will make sure the volatile cache of the storage device -has been flushed before the actual I/O operation is started. This explicitly -guarantees that previously completed write requests are on non-volatile -storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be -set on an otherwise empty bio structure, which causes only an explicit cache -flush without any dependent I/O. It is recommend to use -the blkdev_issue_flush() helper for a pure cache flush. - - -Forced Unit Access ------------------ - -The REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the -filesystem and will make sure that I/O completion for this request is only -signaled after the data has been committed to non-volatile storage. - - -Implementation details for filesystems --------------------------------------- - -Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to -worry if the underlying devices need any explicit cache flushing and how -the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags -may both be set on a single bio. - - -Implementation details for make_request_fn based block drivers --------------------------------------------------------------- - -These drivers will always see the REQ_PREFLUSH and REQ_FUA bits as they sit -directly below the submit_bio interface. For remapping drivers the REQ_FUA -bits need to be propagated to underlying devices, and a global flush needs -to be implemented for bios with the REQ_PREFLUSH bit set. For real device -drivers that do not have a volatile cache the REQ_PREFLUSH and REQ_FUA bits -on non-empty bios can simply be ignored, and REQ_PREFLUSH requests without -data can be completed successfully without doing any work. Drivers for -devices with volatile caches need to implement the support for these -flags themselves without any help from the block layer. - - -Implementation details for request_fn based block drivers --------------------------------------------------------------- - -For devices that do not support volatile write caches there is no driver -support required, the block layer completes empty REQ_PREFLUSH requests before -entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from -requests that have a payload. For devices with volatile write caches the -driver needs to tell the block layer that it supports flushing caches by -doing: - - blk_queue_write_cache(sdkp->disk->queue, true, false); - -and handle empty REQ_OP_FLUSH requests in its prep_fn/request_fn. Note that -REQ_PREFLUSH requests with a payload are automatically turned into a sequence -of an empty REQ_OP_FLUSH request followed by the actual write by the block -layer. For devices that also support the FUA bit the block layer needs -to be told to pass through the REQ_FUA bit using: - - blk_queue_write_cache(sdkp->disk->queue, true, true); - -and the driver must handle write requests that have the REQ_FUA bit set -in prep_fn/request_fn. If the FUA bit is not natively supported the block -layer turns it into an empty REQ_OP_FLUSH request after the actual write. -- cgit v1.2.3-55-g7522