On Wed, 12 Oct 2016 15:19:16 -0400
Zygo Blaxell <ce3g8...@umail.furryterror.org> 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,

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