ReadyNAS 104 - can't reassemble volume

[stardork]

My girlfriend's ReadyNAS had a disk go bad so it was showing as a degraded volume. She pulled the wrong disk out (disk 4 was bad but she accidentally pulled 3). Immediately put the disk back in when she realized it, then restarted the device.

I checked the logs and it shows "Volume: Volume data health changed from Degraded to Dead." at the time the disk was pulled from bay 3. The disks in bays 1, 2, and 3 now show as red and I get the message to remove the inactive volume in order to use the disks. I booted the device in tech support mode and tried to reassemble the volume, but get the following:

mdadm: /dev/md/data-0 assembled from 2 drives - not enough to start the array.
mdadm: /dev/md/1 has been started with 3 drives.
mdadm: /dev/md/0 has been started with 3 drives (out of 4).
mdadm: /dev/md/data-0 assembled from 2 drives - not enough to start the array.

So even though the 3 non-failed disks appear to be healthy, it can only read the raid on 2 of them, so it can't assemble. Any advice on what to try to get it to read the array in all 3 working disks?

Thanks for any suggestions.

[StephenB · Accepted solution]

You need to use the --force (or perhaps --really-force) option to assemble the array.

The instructions (starting from when you log into tech support mode) are

# rnutil chroot
# mdadm --assemble --really-force /dev/md127 /dev/sd[abcd]3
# btrfs device scan
# btrfs fi show
# mount /dev/md127 /data
# ls /data

There could be some data loss - the fact that the array isn't assembling suggests there were cached writes that never made it to the drive she pulled.

[StephenB · Accepted solution]

You need to use the --force (or perhaps --really-force) option to assemble the array.

The instructions (starting from when you log into tech support mode) are

# rnutil chroot
# mdadm --assemble --really-force /dev/md127 /dev/sd[abcd]3
# btrfs device scan
# btrfs fi show
# mount /dev/md127 /data
# ls /data

There could be some data loss - the fact that the array isn't assembling suggests there were cached writes that never made it to the drive she pulled.

[stardork]

Looks like it worked, thanks so much!