On Wed, 12 Oct 2016 15:19:16 -0400 Zygo Blaxell <ce3g8jdj@xxxxxxxxxxxxxxxxxxxxx> wrote: > I'm not even sure btrfs does this--I haven't checked precisely what > it does in dup mode. It could send both copies of metadata to the > disks with a single barrier to separate both metadata updates from > the superblock updates. That would be bad in this particular case. It would be bad in any case, including a single physical disk and no RAID, and I don't think there's any basis to speculate that mdadm doesn't implement write barriers properly. > In degraded RAID5/6 mode, all writes temporarily corrupt data, so if there > is an interruption (system crash, a disk times out, etc) in degraded mode, Moreover, in any non-COW system writes temporarily corrupt data. So again, writing to a (degraded or not) mdadm RAID5 is not much different than writing to a single physical disk. However I believe in the Btrfs case metadata is always COW, so this particular problem may be not as relevant here in the first place. -- With respect, Roman
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