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

[Martin Steigerwald <martin.steigerwald@teamix.de>]

Am Mittwoch, 16. November 2016, 15:43:36 CET schrieb Roman Mamedov:
> On Wed, 16 Nov 2016 11:25:00 +0100
> 
> Martin Steigerwald <martin.steigerwald@teamix.de> wrote:
> >     merkaba:~> mount -o degraded,clear_cache /dev/satafp1/backup /mnt/zeit
> >     mount: Falscher Dateisystemtyp, ungültige Optionen, der
> >     Superblock von /dev/mapper/satafp1-backup ist beschädigt, fehlende
> >     Kodierungsseite oder ein anderer Fehler
> >     
> >           Manchmal liefert das Systemprotokoll wertvolle Informationen –
> >           versuchen Sie  dmesg | tail  oder ähnlich
> >     
> >     merkaba:~#32> dmesg | tail -6
> >     [ 3080.120687] BTRFS info (device dm-13): allowing degraded mounts
> >     [ 3080.120699] BTRFS info (device dm-13): force clearing of disk cache
> >     [ 3080.120703] BTRFS info (device dm-13): disk space caching is
> >     enabled
> >     [ 3080.120706] BTRFS info (device dm-13): has skinny extents
> >     [ 3080.150957] BTRFS warning (device dm-13): missing devices (1)
> >     exceeds the limit (0), writeable mount is not allowed
> >     [ 3080.195941] BTRFS: open_ctree failed
> 
> I have to wonder did you read the above message? What you need at this point
> is simply "-o degraded,ro". But I don't see that tried anywhere down the
> line.
> 
> See also (or try): https://patchwork.kernel.org/patch/9419189/

Actually I read that one, but I read more into it than what it was saying:

I read into it that BTRFS would automatically use a read only mount.


merkaba:~> mount -o degraded,ro /dev/satafp1/daten /mnt/zeit

actually really works. *Thank you*, Roman.


I do think that above kernel messages invite such a kind of interpretation
tough. I took the "BTRFS: open_ctree failed" message as indicative to some
structural issue with the filesystem.


So mounting work although for some reason scrubbing is aborted (I had this
issue a long time ago on my laptop as well). After removing /var/lib/btrfs 
scrub status file for the filesystem:

    merkaba:~> btrfs scrub start /mnt/zeit
    scrub started on /mnt/zeit, fsid […] (pid=9054)
    merkaba:~> btrfs scrub status /mnt/zeit
    scrub status for […]
            scrub started at Wed Nov 16 11:52:56 2016 and was aborted after 
00:00:00
            total bytes scrubbed: 0.00B with 0 errors

Anyway, I will now just rsync off the files.

Interestingly enough btrfs restore complained about looping over certain
files… lets see whether the rsync or btrfs send/receive proceeds through.

Ciao,

-- 
Martin Steigerwald  | Trainer

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