Troubleshooting RAID Problems

If you want to view the status of your RAID array, you can check the contents of the /proc/mdstat file:

$ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md_d0 : active raid1 sdb1[0]
      976759936 blocks [2/1] [U_]

unused devices: <none>

The highlighted part shows that the array is:

Active
Running as a "raid1" array
Currently the only device in the array is /dev/sdb1 and it is operating correctly (active)

Additional output can also be gained by running:

mdadm --detail <RAID Device>

For example:

$ mdadm --detail /dev/md0
/dev/md0:
             Version : 00.90
  Creation Time : Sat Feb 19 16:29:11 2011
        Raid Level : raid1
         Array Size : 976759936 (931.51 GiB 1000.20 GB)
  Used Dev Size : 976759936 (931.51 GiB 1000.20 GB)
     Raid Devices : 2
    Total Devices : 2
 Preferred Minor : 0
       Persistence : Superblock is persistent

    Update Time : Mon Feb 21 12:42:12 2011
                State : clean, degraded
  Active Devices : 1
Working Devices: 1
  Failed Devices : 1
  Spare Devices : 0

           UUID : a1500c81:a4bdadeb:382ce4fd:28800507 (local to host homeServer)
         Events : 0.72

    Number   Major   Minor   RaidDevice State
       0       8       17        0      active sync   /dev/sdb1
       1       0        0         1      removed

       2       8       33         -      faulty spare   /dev/sdc1

The above example shows that member /dev/sdb1 is the currently active device and that it's mirror, /dev/sdc1, has failed (-listed as faulty).