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mdadm: Add description of write journal to md.4 

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: NeilBrown <neilb@suse.com>
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Song Liu 2015-10-21 11:35:15 -07:00 committed by NeilBrown
parent 051f326550
commit 28f83f6d3b
1 changed files with 20 additions and 0 deletions
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@ -874,6 +874,26 @@ The list is particularly useful when recovering to a spare. If a few blocks
cannot be read from the other devices, the bulk of the recovery can
complete and those few bad blocks will be recorded in the bad block list.
.SS RAID456 WRITE JOURNAL
Due to non-atomicity nature of RAID write operations, interruption of
write operations (system crash, etc.) to RAID456 array can lead to
inconsistent parity and data loss (so called RAID-5 write hole).
To plug the write hole, from Linux 4.4 (to be confirmed),
.I md
supports write ahead journal for RAID456. When the array is created,
an additional journal device can be added to the array through
.IR write-journal
option. The RAID write journal works similar to file system journals.
Before writing to the data disks, md persists data AND parity of the
stripe to the journal device. After crashes, md searches the journal
device for incomplete write operations, and replay them to the data
disks.
When the journal device fails, the RAID array is forced to run in
read-only mode.
.SS WRITE-BEHIND
From Linux 2.6.14,