OK, so you have that nice and shiny server up and running with a RAID array, and all of a sudden you start getting ‘Degraded Array’ messages…
Assuming the error is not hardware-related (i.e. one drive of the array that is failing) you can easily recover from this message.
Examine the current RAID status
# cat /proc/mdstat
A valid RAID will look like this:
md1 : active raid1 sdb1[1] sda1[0]
104320 blocks [2/2] [UU]
An invalid RAID entry will look like:
md2 : active raid1 sda2[0]
78043648 blocks [2/1] [U_]
Examine the failing entry
# mdadm -D /dev/md2
will return you the status messages including:
Number Major Minor RaidDevice State 0 8 2 0 active sync /dev/sda2 1 0 0 -1 removed
Only /dev/sda2 is in active sync.
Aha, it seems that /dev/sdb2 is missing from the array. Let’s add it again
# mdadm /dev/md2 -a /dev/sdb2
Reverify the RAID status
# cat /proc/mdstat
Returns
Personalities : [raid1]
md2 : active raid1 sdb2[2] sda2[0]
78043648 blocks [2/1] [U_]
[=====>...............]
recovery = 28.3% (22156672/78043648)
finish=36.9min speed=25179K/sec
md1 : active raid1 sdb1[1] sda1[0]
104320 blocks [2/2] [UU]
OK, the system is now rebuilding the md2 entry.
This can also be verified with
# mdadm -D /dev/md2
that will return
/dev/md2:
Version : 00.90.01
Creation Time : Tue Apr 11 21:07:15 2006
Raid Level : raid1
Array Size : 78043648 (74.43 GiB 79.92 GB)
Device Size : 78043648 (74.43 GiB 79.92 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 2
Persistence : Superblock is persistent
Update Time : Sat Nov 4 10:55:33 2006
State : clean, degraded, recovering
Active Devices : 1
Working Devices : 2
Failed Devices : 0
Spare Devices : 1
Rebuild Status : 30% complete
Number Major Minor RaidDevice State
0 8 2 0 active sync /dev/sda2
1 0 0 -1 removed
2 8 18 1 spare /dev/sdb2
UUID : e151ccd0:5bb92b79:d26db88d:0581e61d
Events : 0.4130158
Let it finish and you are up and running again…