Re: raid1 degraded mount still produce single chunks, writeable mount not allowed

[Adam Borowski <kilobyte@angband.pl>]

On Wed, Mar 01, 2017 at 05:30:37PM -0700, Chris Murphy wrote:
> [1717713.408675] BTRFS warning (device dm-8): missing devices (1)
> exceeds the limit (0), writeable mount is not allowed
> [1717713.446453] BTRFS error (device dm-8): open_ctree failed
> 
> [chris@f25s ~]$ uname -r
> 4.9.8-200.fc25.x86_64
> 
> I thought this was fixed. I'm still getting a one time degraded rw
> mount, after that it's no longer allowed, which really doesn't make
> any sense because those single chunks are on the drive I'm trying to
> mount.

Well, there's Qu's patch at:
https://www.spinics.net/lists/linux-btrfs/msg47283.html
but it doesn't apply cleanly nor is easy to rebase to current kernels.

> I don't understand what problem this proscription is trying to
> avoid. If it's OK to mount rw,degraded once, then it's OK to allow it
> twice. If it's not OK twice, it's not OK once.

Well, yeah.  The current check is naive and wrong.  It does have a purpose,
just fails in this, very common, case.

For people needing to recover their filesystem at this moment there's
https://www.spinics.net/lists/linux-btrfs/msg62473.html
but it removes the protection you still want for other cases.

This problem pops up way too often, thus I guess that if not the devs, then
at least us in the peanut gallery should do the work reviving the real
solution.  Obviously, I for one am shortish on tuits at the moment...

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