Linux – mdadm – Recovering a ‘split’ RAID1 array

linuxmdadmraid

I have two drives that used to be part of a single RAID1 volume but it appears that one of them went offline for some time, something I've noticed just now when I rebooted my system.

I now seem to have two RAID volumes, as reported by:

# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md126 : active raid1 sdc[1]
      2096116 blocks super 1.2 [2/1] [_U]

md127 : active (auto-read-only) raid1 sdb[0]
      2096116 blocks super 1.2 [2/1] [U_]

unused devices: <none>

Not exactly sure where to go from here. How can I merge and re-sync these volumes without data loss?

Thanks.

Best Answer

First of all you have to find out which of the two devices has the more recent copy of the RAID. Try to mount them read-only and look at the files. The logs (esp. /var/log/syslog*) might help here too. Then you have to verify that the drive that went offline isn't broken (logs, again, and SMART).

Then kill the other raid (mdadm --fail, mdadm --remove) and add the device to the correct one. This will trigger a resync, but then you should have a fine raid again.

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