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| -rw-r--r-- | docs/throttle.txt | 108 |
1 files changed, 107 insertions, 1 deletions
diff --git a/docs/throttle.txt b/docs/throttle.txt index cd4e109d39..b5b78b7326 100644 --- a/docs/throttle.txt +++ b/docs/throttle.txt @@ -1,6 +1,6 @@ The QEMU throttling infrastructure ================================== -Copyright (C) 2016 Igalia, S.L. +Copyright (C) 2016,2020 Igalia, S.L. Author: Alberto Garcia <berto@igalia.com> This work is licensed under the terms of the GNU GPL, version 2 or @@ -253,3 +253,109 @@ up. After those 60 seconds the bucket will have leaked 60 x 100 = Also, due to the way the algorithm works, longer burst can be done at a lower I/O rate, e.g. 1000 IOPS during 120 seconds. + + +The 'throttle' block filter +--------------------------- +Since QEMU 2.11 it is possible to configure the I/O limits using a +'throttle' block filter. This filter uses the exact same throttling +infrastructure described above but can be used anywhere in the node +graph, allowing for more flexibility. + +The user can create an arbitrary number of filters and each one of +them must be assigned to a group that contains the actual I/O limits. +Different filters can use the same group so the limits are shared as +described earlier in "Applying I/O limits to groups of disks". + +A group can be created using the object-add QMP function: + + { "execute": "object-add", + "arguments": { + "qom-type": "throttle-group", + "id": "group0", + "props": { + "limits" : { + "iops-total": 1000 + "bps-write": 2097152 + } + } + } + } + +throttle-group has a 'limits' property (of type ThrottleLimits as +defined in qapi/block-core.json) which can be set on creation or later +with 'qom-set'. + +A throttle-group can also be created with the -object command line +option but at the moment there is no way to pass a 'limits' parameter +that contains a ThrottleLimits structure. The solution is to set the +individual values directly, like in this example: + + -object throttle-group,id=group0,x-iops-total=1000,x-bps-write=2097152 + +Note however that this is not a stable API (hence the 'x-' prefixes) and +will disappear when -object gains support for structured options and +enables use of 'limits'. + +Once we have a throttle-group we can use the throttle block filter, +where the 'file' property must be set to the block device that we want +to filter: + + { "execute": "blockdev-add", + "arguments": { + "options": { + "driver": "qcow2", + "node-name": "disk0", + "file": { + "driver": "file", + "filename": "/path/to/disk.qcow2" + } + } + } + } + + { "execute": "blockdev-add", + "arguments": { + "driver": "throttle", + "node-name": "throttle0", + "throttle-group": "group0", + "file": "disk0" + } + } + +A similar setup can also be done with the command line, for example: + + -drive driver=throttle,throttle-group=group0, + file.driver=qcow2,file.file.filename=/path/to/disk.qcow2 + +The scenario described so far is very simple but the throttle block +filter allows for more complex configurations. For example, let's say +that we have three different drives and we want to set I/O limits for +each one of them and an additional set of limits for the combined I/O +of all three drives. + +First we would define all throttle groups, one for each one of the +drives and one that would apply to all of them: + + -object throttle-group,id=limits0,x-iops-total=2000 + -object throttle-group,id=limits1,x-iops-total=2500 + -object throttle-group,id=limits2,x-iops-total=3000 + -object throttle-group,id=limits012,x-iops-total=4000 + +Now we can define the drives, and for each one of them we use two +chained throttle filters: the drive's own filter and the combined +filter. + + -drive driver=throttle,throttle-group=limits012, + file.driver=throttle,file.throttle-group=limits0 + file.file.driver=qcow2,file.file.file.filename=/path/to/disk0.qcow2 + -drive driver=throttle,throttle-group=limits012, + file.driver=throttle,file.throttle-group=limits1 + file.file.driver=qcow2,file.file.file.filename=/path/to/disk1.qcow2 + -drive driver=throttle,throttle-group=limits012, + file.driver=throttle,file.throttle-group=limits2 + file.file.driver=qcow2,file.file.file.filename=/path/to/disk2.qcow2 + +In this example the individual drives have IOPS limits of 2000, 2500 +and 3000 respectively but the total combined I/O can never exceed 4000 +IOPS. |