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authorHeinz Mauelshagen2016-11-30 22:31:05 +0100
committerMike Snitzer2017-01-25 12:49:06 +0100
commit63c32ed4afc2afd6b5551a8fcdea5b546dcaca4f (patch)
tree8cdb6bf0a8080912d2590a3d5513b23b8c51ddd1 /Documentation/device-mapper
parentdm raid: be prepared to accept arbitrary '- -' tuples (diff)
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dm raid: add raid4/5/6 journaling support
Add md raid4/5/6 journaling support (upstream commit bac624f3f86a started the implementation) which closes the write hole (i.e. non-atomic updates to stripes) using a dedicated journal device. Background: raid4/5/6 stripes hold N data payloads per stripe plus one parity raid4/5 or two raid6 P/Q syndrome payloads in an in-memory stripe cache. Parity or P/Q syndromes used to recover any data payloads in case of a disk failure are calculated from the N data payloads and need to be updated on the different component devices of the raid device. Those are non-atomic, persistent updates. Hence a crash can cause failure to update all stripe payloads persistently and thus cause data loss during stripe recovery. This problem gets addressed by writing whole stripe cache entries (together with journal metadata) to a persistent journal entry on a dedicated journal device. Only if that journal entry is written successfully, the stripe cache entry is updated on the component devices of the raid device (i.e. writethrough type). In case of a crash, the entry can be recovered from the journal and be written again thus ensuring consistent stripe payload suitable to data recovery. Future dependencies: once writeback caching being worked on to compensate for the throughput implictions involved with writethrough overhead is supported with journaling in upstream, an additional patch based on this one will support it in dm-raid. Journal resilience related remarks: because stripes are recovered from the journal in case of a crash, the journal device better be resilient. Resilience becomes mandatory with future writeback support, because loosing the working set in the log means data loss as oposed to writethrough, were the loss of the journal device 'only' reintroduces the write hole. Fix comment on data offsets in parse_dev_params() and initialize new_data_offset as well. Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Diffstat (limited to 'Documentation/device-mapper')
-rw-r--r--Documentation/device-mapper/dm-raid.txt13
1 files changed, 13 insertions, 0 deletions
diff --git a/Documentation/device-mapper/dm-raid.txt b/Documentation/device-mapper/dm-raid.txt
index c84cdd220f9d..0d199353e477 100644
--- a/Documentation/device-mapper/dm-raid.txt
+++ b/Documentation/device-mapper/dm-raid.txt
@@ -161,6 +161,15 @@ The target is named "raid" and it accepts the following parameters:
the RAID type (i.e. the allocation algorithm) as well, e.g.
changing from raid5_ls to raid5_n.
+ [journal_dev <dev>]
+ This option adds a journal device to raid4/5/6 raid sets and
+ uses it to close the 'write hole' caused by the non-atomic updates
+ to the component devices which can cause data loss during recovery.
+ The journal device is used as writethrough thus causing writes to
+ be throttled versus non-journaled raid4/5/6 sets.
+ Takeover/reshape is not possible with a raid4/5/6 journal device;
+ it has to be deconfigured before requesting these.
+
<#raid_devs>: The number of devices composing the array.
Each device consists of two entries. The first is the device
containing the metadata (if any); the second is the one containing the
@@ -245,6 +254,9 @@ recovery. Here is a fuller description of the individual fields:
<data_offset> The current data offset to the start of the user data on
each component device of a raid set (see the respective
raid parameter to support out-of-place reshaping).
+ <journal_char> 'A' - active raid4/5/6 journal device.
+ 'D' - dead journal device.
+ '-' - no journal device.
Message Interface
@@ -318,3 +330,4 @@ Version History
fails reading a superblock. Correctly emit 'maj:min1 maj:min2' and
'D' on the status line. If '- -' is passed into the constructor, emit
'- -' on the table line and '-' as the status line health character.
+1.10.0 Add support for raid4/5/6 journal device