How do I reactivate my MDADM RAID5 array?

I've just moved house which involved dismantling my server and re-connecting it. Since doing so, one of my MDADM RAID5 arrays is appearing as inactive:

root@mserver:/tmp# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10]
md1 : active raid5 sdc1[1] sdh1[2] sdg1[0] 3907023872 blocks level 5, 64k chunk, algorithm 2 [3/3] [UUU]
md0 : inactive sdd1[0](S) sdf1[3](S) sde1[2](S) sdb1[1](S) 3907039744 blocks
unused devices: <none>

It looks to me as though it's found all of the disks but for some reason doesn't want to use them.

So what do the (S) labels mean and how can I tell MDADM to start using the array again?

[Edit] I just tried stopping and assembling the array with -v:

root@mserver:~# mdadm --stop /dev/md0
mdadm: stopped /dev/md0
root@mserver:~# mdadm --assemble --scan -v
mdadm: /dev/sde1 is identified as a member of /dev/md0, slot 2.
mdadm: /dev/sdf1 is identified as a member of /dev/md0, slot 3.
mdadm: /dev/sdd1 is identified as a member of /dev/md0, slot 0.
mdadm: /dev/sdb1 is identified as a member of /dev/md0, slot 1.
mdadm: added /dev/sdd1 to /dev/md0 as 0 (possibly out of date)
mdadm: added /dev/sdb1 to /dev/md0 as 1 (possibly out of date)
mdadm: added /dev/sdf1 to /dev/md0 as 3 (possibly out of date)
mdadm: added /dev/sde1 to /dev/md0 as 2
mdadm: /dev/md0 assembled from 1 drive - not enough to start the array.

..and entering cat /proc/mdstat looks no different.

[Edit2] Not sure if it helps but this is the result of examining each disk:

root@mserver:~# mdadm --examine /dev/sdb1

/dev/sdb1: Magic : a92b4efc Version : 0.90.00 UUID : 2f331560:fc85feff:5457a8c1:6e047c67 (local to host mserver) Creation Time : Sun Feb 1 20:53:39 2009 Raid Level : raid5 Used Dev Size : 976759936 (931.51 GiB 1000.20 GB) Array Size : 2930279808 (2794.53 GiB 3000.61 GB) Raid Devices : 4 Total Devices : 4
Preferred Minor : 0 Update Time : Sat Apr 20 13:22:27 2013 State : clean Active Devices : 4
Working Devices : 4 Failed Devices : 0 Spare Devices : 0 Checksum : 6c8f71a3 - correct Events : 955190 Layout : left-symmetric Chunk Size : 64K Number Major Minor RaidDevice State
this 1 8 17 1 active sync /dev/sdb1 0 0 8 113 0 active sync /dev/sdh1 1 1 8 17 1 active sync /dev/sdb1 2 2 8 97 2 active sync /dev/sdg1 3 3 8 33 3 active sync /dev/sdc1

root@mserver:~# mdadm --examine /dev/sdd1

/dev/sdd1: Magic : a92b4efc Version : 0.90.00 UUID : 2f331560:fc85feff:5457a8c1:6e047c67 (local to host mserver) Creation Time : Sun Feb 1 20:53:39 2009 Raid Level : raid5 Used Dev Size : 976759936 (931.51 GiB 1000.20 GB) Array Size : 2930279808 (2794.53 GiB 3000.61 GB) Raid Devices : 4 Total Devices : 2
Preferred Minor : 0 Update Time : Sat Apr 20 18:37:23 2013 State : active Active Devices : 2
Working Devices : 2 Failed Devices : 2 Spare Devices : 0 Checksum : 6c812869 - correct Events : 955205 Layout : left-symmetric Chunk Size : 64K Number Major Minor RaidDevice State
this 0 8 113 0 active sync /dev/sdh1 0 0 8 113 0 active sync /dev/sdh1 1 1 0 0 1 faulty removed 2 2 8 97 2 active sync /dev/sdg1 3 3 0 0 3 faulty removed

root@mserver:~# mdadm --examine /dev/sde1

/dev/sde1: Magic : a92b4efc Version : 0.90.00 UUID : 2f331560:fc85feff:5457a8c1:6e047c67 (local to host mserver) Creation Time : Sun Feb 1 20:53:39 2009 Raid Level : raid5 Used Dev Size : 976759936 (931.51 GiB 1000.20 GB) Array Size : 2930279808 (2794.53 GiB 3000.61 GB) Raid Devices : 4 Total Devices : 2
Preferred Minor : 0 Update Time : Sun Apr 21 14:00:43 2013 State : clean Active Devices : 1
Working Devices : 1 Failed Devices : 2 Spare Devices : 0 Checksum : 6c90cc70 - correct Events : 955219 Layout : left-symmetric Chunk Size : 64K Number Major Minor RaidDevice State
this 2 8 97 2 active sync /dev/sdg1 0 0 0 0 0 removed 1 1 0 0 1 faulty removed 2 2 8 97 2 active sync /dev/sdg1 3 3 0 0 3 faulty removed

root@mserver:~# mdadm --examine /dev/sdf1

/dev/sdf1: Magic : a92b4efc Version : 0.90.00 UUID : 2f331560:fc85feff:5457a8c1:6e047c67 (local to host mserver) Creation Time : Sun Feb 1 20:53:39 2009 Raid Level : raid5 Used Dev Size : 976759936 (931.51 GiB 1000.20 GB) Array Size : 2930279808 (2794.53 GiB 3000.61 GB) Raid Devices : 4 Total Devices : 4
Preferred Minor : 0 Update Time : Sat Apr 20 13:22:27 2013 State : clean Active Devices : 4
Working Devices : 4 Failed Devices : 0 Spare Devices : 0 Checksum : 6c8f71b7 - correct Events : 955190 Layout : left-symmetric Chunk Size : 64K Number Major Minor RaidDevice State
this 3 8 33 3 active sync /dev/sdc1 0 0 8 113 0 active sync /dev/sdh1 1 1 8 17 1 active sync /dev/sdb1 2 2 8 97 2 active sync /dev/sdg1 3 3 8 33 3 active sync /dev/sdc1

I have some notes which suggest the drives were originally assembled as follows:

md0 : active raid5 sdb1[1] sdc1[3] sdh1[0] sdg1[2] 2930279808 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]

[Edit3]

Looking through the log it looks like the following happened (based on the Update Time in the --examine results):

1. sdb and sdf were knocked out some time after 13:22 on the 20th
2. sdd was knocked out some time after 18:37 on the 20th
3. the server was shut down some time after 14:00 on the 1st

Given that two disks went down (apparently) simultaneously I think it should be reasonably safe to assume the array wouldn't have been written to after that point(?) and so it should be relatively safe to force it to re-instate in the correct order? What's the safest command to do that with and is there a way to do it without writing any changes?

3 Answers

The S labels means the disk is regarded as "spare". You should try stopping and re-starting the array:

 mdadm --stop /dev/md0 mdadm --assemble --scan

to re-assemble the array and if that doesn't work, you may need to update your mdadm.conf, see for example this question for details on how to do that.

This question is a bit old, but the answer might help someone facing a similar situation. Looking at the event counts from the mdadm --examine output you have provided, they seem close enough (955190 - for sdb1 and sdf1, 955219 for sde1 and for sdd1 you've got 955205). If they are below 40-50, this is OK, and in that case the recommended course of action is to assemble your array manually, forcing mdadm to accept the drives despite the event count difference:

Stop the array:

mdadm --stop /dev/md0

Then try to reassemble the array manually:

mdadm --assemble --force /dev/md0 /dev/sdb1 /dev/sdd1 /dev/sde1 /dev/sdf1

Check the status of the array, to examine if the drive list/structure is OK (bottom of command output will show which drive is at what status and at which position in the array):

mdadm --detail /dev/md0

If the structure is OK, check rebuilding progress:

cat /proc/mdstat

You can activate Raid md0 with the below command

mdadm -A /dev/md0

and this command to update mdadm.conf file

mdadm --examine --scan >> /etc/mdadm/mdadm.conf