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I am new to raid, and set up a software RAID1 with two sata disk a month ago. I didn't check the status, but tested by unplugging one of the drives and verifying the system worked normally.
Today, I saw a mail in the terminal about raid failure.

Subject: DegradedArray event on /dev/md/1:ws

looking at /var/mail, I saw similar messages from few months back, which I never noticed. The drives are quite new, and I expect the problem in my configuration. I read somewhere that the underscore in the below status hows a removed drive, but the status of individual array do not show any failed ones.
What could be causing this?

cat /proc/mdstat
 Personalities : [raid1] 
  md1 : active raid1 sda6[0]
  471572344 blocks super 1.2 [2/1] [U_]

 md0 : active raid1 sda1[0]
  975860 blocks super 1.2 [2/1] [U_]

unused devices: <none>

mdadm -D /dev/md0
/dev/md0:
    Version : 1.2  
Creation Time : Mon Mar 14 21:14:03 2011
 Raid Level : raid1
 Array Size : 975860 (953.15 MiB 999.28 MB)
Used Dev Size : 975860 (953.15 MiB 999.28 MB)
 Raid Devices : 2
Total Devices : 1
Persistence : Superblock is persistent

Update Time : Wed Apr  6 05:21:36 2011
      State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
 Spare Devices : 0

       Name : ws:0  (local to host ws)
       UUID : c6d0fb74:948f791d:7dbebe82:9c11f022
     Events : 314

Number   Major   Minor   RaidDevice State
   0       8        1        0      active sync   /dev/sda1
   1       0        0        1      removed

mdadm -D /dev/md1
/dev/md1:
    Version : 1.2
Creation Time : Mon Mar 14 21:14:35 2011
 Raid Level : raid1
 Array Size : 471572344 (449.73 GiB 482.89 GB)
Used Dev Size : 471572344 (449.73 GiB 482.89 GB)
Raid Devices : 2
Total Devices : 1
Persistence : Superblock is persistent

Update Time : Wed Apr  6 05:51:59 2011
      State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0

       Name : ws:1  (local to host ws)
       UUID : 3df658dc:0f252703:065ec271:3492458b
     Events : 188462

Number   Major   Minor   RaidDevice State
   0       8        6        0      active sync   /dev/sda6
   1       0        0        1      removed
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1 Answer 1

up vote 4 down vote accepted

Did you perhaps forget to re-add the drive to the array when you plugged it back in?
http://www.howtoforge.com/replacing_hard_disks_in_a_raid1_array has more information / tutorials on this.

For the future, when you "test" a RAID array (hardware or software), simply failing a drive is not sufficient: You must verify that the array rebuilds and comes out of error/degraded status to have a complete test (and some people would even say you have to go the extra step and fail/rebuild the other disk too).

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thanks for ur help... –  bsreekanth Apr 6 '11 at 18:28
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