Forum Discussion

Tutor

Jul 20, 2016

Solved

btrfs_merge_bio_hook+a0 error, recovery possible?

Hi, I have a ReadyNAS 204 with the btrfs_merge_bio_hook+a0 error. After reboot I can ssh into the box, but the web interface does not load and the data volume is not mounted. Output of cat /p...

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edkedk
Aug 01, 2016
Eventually I used --path-regex to restore specific directories and it worked fine. Seems that everything I wanted was copied.

mdgm-ntgr

NETGEAR Employee Retired

Jul 21, 2016

Shell commands would have to be used to try and mount it or attempt to recover the data and these could just as easily be done via SSH on the RN204. However attempting to do that would be at your own risk.

Have you checked the smart stats of the disks for SMART errors?

edkedk

Tutor

Jul 21, 2016

I just checked smart values using smartctl and see no error on any disk.

I'd like to try to manually mount the volume. Could you assist me with the required shell commands?

Hopchen
Prodigy
Jul 21, 2016
Hi,

As mdgm said, running commands from the terminal can be "dangerous" if you are not experienced/comfortable with Linux and BTRFS. So, just be careful :)

I think you should wait mounting the volume. First check that it is not something benign (like a 100% full root/OS). Post the output of the command:
df -h

If the OS is not full, then I would suggest you do a read-only filesystem check on the data volume before attempting the mount. You can do this by running the command:
btrfsck /dev/md127

That might take some time. Please post the result of the FS check. The above command will not try to repair anything, it will just give us info about the state of the filesystem and the command can only be run on a non-mounted filesystem (so don't attempt to mount first).

Thanks
- edkedk
  Tutor
  Jul 21, 2016
  I have some Linux experience but none with btrfs.
  
  # df -h Filesystem Size Used Avail Use% Mounted on tmpfs 10M 4.0K 10M 1% /dev /dev/md0 3.7G 923M 2.6G 26% / tmpfs 1011M 0 1011M 0% /dev/shm tmpfs 1011M 500K 1011M 1% /run tmpfs 1011M 0 1011M 0% /sys/fs/cgroup tmpfs 1011M 0 1011M 0% /media
  # btrfsck /dev/md127 Couldn't open file system
  - Hopchen
    Prodigy
    Jul 21, 2016
    And the RAID is definitely still running?
    
    cat /proc/mdstat