Repairing a RAID5 software array (Debian Linux)

RAID5 software arrays in Linux are very simple to setup and should run smoothly. Inevitably something will go wrong – but luckily that’s why we use RAID5 – there should be no data loss.

Software RAID HOWTO is the best reference.

Check the RAID

Identify the problematic disk [UU_]

mdadm --detail /dev/md0
cat /proc/mdstat

Remove failed disk

mdadm /dev/md0 -r /dev/hdg1

Add new disk

mdadm /dev/md0 -a /dev/hdg1

Once rebuilding has started you can check the status via /proc/mdstat (depending on the array size it may take quite a long time).

cat /proc/mdstat
Personalities : [raid5]
md0 : active raid5 hdg1[3] hdc1[0] hde1[1]
      488391680 blocks level 5, 128k chunk, algorithm 2 [3/2] [UU_]
      [>....................]  recovery =  0.8% (2098944/244195840) finish=205.3min speed=19648K/sec
unused devices: <none>