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?



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.

| improve this answer | |
  • The disks/partitions that are showing "U" are the ones that will be most up-to-date. There is no need to check the now out-of-date disks as mdadm knows which one is most current. – Alastair McCormack Nov 8 '12 at 8:51

You should just be able to re-add the disk with the -a option. For example:

mdadm -a /dev/md127 /dev/sdc1

Use mdadm -D /dev/mddevice to get more information about the array. You may need to use -r option first to remove the problematic disk partition before re-adding

| improve this answer | |
  • I had to do mdadm --stop /dev/md126 before adding it again – Claes Mogren Feb 9 '13 at 15:15

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