Re: degraded BTRFS RAID 1 not mountable: open_ctree failed, unable to find block group for 0

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-11-17 15:05, Chris Murphy wrote:
> I think the wiki should be updated to reflect that raid1 and raid10
> are mostly OK. I think it's grossly misleading to consider either as
> green/OK when a single degraded read write mount creates single chunks
> that will then prevent a subsequent degraded read write mount. And
> also the lack of various notifications of device faultiness I think
> make it less than OK also. It's not in the "do not use" category but
> it should be in the middle ground status so users can make informed
> decisions.
>
It's worth pointing out also regarding this:
* This is handled sanely in recent kernels (the check got changed from 
per-fs to per-chunk, so you still have a usable FS if all the single 
chunks are only on devices you still have).
* This is only an issue with filesystems with exactly two disks.  If a 
3+ disk raid1 FS goes degraded, you still generate raid1 chunks.
* There are a couple of other cases where raid1 mode falls flat on it's 
face (lots of I/O errors in a short span of time with compression 
enabled can cause a kernel panic for example).
* raid10 has some other issues of it's own (you lose two devices, your 
filesystem is dead, which shouldn't be the case 100% of the time (if you 
lose different parts of each mirror, BTRFS _should_ be able to recover, 
it just doesn't do so right now)).

As far as the failed device handling issues, those are a problem with 
BTRFS in general, not just raid1 and raid10, so I wouldn't count those 
against raid1 and raid10.