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author | Song Liu <songliubraving@fb.com> | 2015-10-21 11:35:15 -0700 |
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committer | NeilBrown <neilb@suse.com> | 2015-10-22 12:19:11 +1100 |
commit | 28f83f6d3b1b3b386e60b45848b9284e2238f77f (patch) | |
tree | 07e62de02b1a84ff6bb0c77a8ef61537b33c45b5 /md.4 | |
parent | 051f326550f5f2c6e3329878d2c9a5558a5a41ff (diff) | |
download | mdadm-28f83f6d3b1b3b386e60b45848b9284e2238f77f.tar.gz |
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>
Diffstat (limited to 'md.4')
-rw-r--r-- | md.4 | 20 |
1 files changed, 20 insertions, 0 deletions
@@ -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, |