Btrfs unmountable - claims it is missing a volume

Btrfs unmountable - claims it is missing a volume

[Grdykopląs Namorzyn]

Hello,

While my btrfs mounts ok on several instances of my home OpenElec, but
on ArchLinux it fails with:

[174828.954975] BTRFS error (device sdc): super_num_devices 3 mismatch
with num_devices 2 found here

[174828.956969] BTRFS error (device sdc): failed to read chunk tree: -22

Note that it never had 3 volumes! Same happens with -o degraded:

[174828.860334] BTRFS info (device sdc): allowing degraded mounts
[174828.860352] BTRFS info (device sdc): disk space caching is enabled
[174828.954975] BTRFS error (device sdc): super_num_devices 3 mismatch
with num_devices 2 found here
[174828.956969] BTRFS error (device sdc): failed to read chunk tree: -22

My system:

[root@storage ~]#   uname -a
Linux storage 4.7.2-1-ARCH #1 SMP PREEMPT Sat Aug 20 23:02:56 CEST
2016 x86_64 GNU/Linux
[root@storage ~]#   btrfs --version
btrfs-progs v4.7
[root@storage ~]#   btrfs fi show
Label: none  uuid: 8ef88ab7-0dae-481c-bb68-5fbed7e86efa
        Total devices 3 FS bytes used 4.68TiB
        devid    1 size 3.64TiB used 3.26TiB path /dev/sdc
        devid    3 size 1.82TiB used 1.44TiB path /dev/sdb
        *** Some devices missing

What can I do to mount it on this machine?

Re: Btrfs unmountable - claims it is missing a volume

[David Sterba]

Hi,

On Sat, Sep 10, 2016 at 08:59:21AM +0200, Grdykopląs Namorzyn wrote:
> Hello,
> 
> While my btrfs mounts ok on several instances of my home OpenElec, but
> on ArchLinux it fails with:
> 
> [174828.954975] BTRFS error (device sdc): super_num_devices 3 mismatch
> with num_devices 2 found here

I got another report of the same problem (2 devices expected but really
there was just 1). The refused mount is caused by additional checks [1]
that are in 4.7 but were not in previous versions. The fix was to set
the superblock::num_devices to the right number (after some
verification).  I'll add the repair capability to checker, let me know
if you'd use the one-time fixup utility. Otherwise, the cause why device
numbers are off is unknown.

[1] https://git.kernel.org/linus/99e3ecfcb9f4ca35192d20a5bea158b81f600062