How do I stop and repair a RAID 5 array that has failed and has I/O pending?
- by Ben Hymers
The short version: I have a failed RAID 5 array which has a bunch of processes hung waiting on I/O operations on it; how can I recover from this?
The long version: Yesterday I noticed Samba access was being very sporadic; accessing the server's shares from Windows would randomly lock up explorer completely after clicking on one or two directories. I assumed it was Windows being a pain and left it. Today the problem is the same, so I did a little digging; the first thing I noticed was that running ps aux | grep smbd gives a lot of lines like this:
ben 969 0.0 0.2 96088 4128 ? D 18:21 0:00 smbd -F
root 1708 0.0 0.2 93468 4748 ? Ss 18:44 0:00 smbd -F
root 1711 0.0 0.0 93468 1364 ? S 18:44 0:00 smbd -F
ben 3148 0.0 0.2 96052 4160 ? D Mar07 0:00 smbd -F
...
There are a lot of processes stuck in the "D" state. Running ps aux | grep " D" shows up some other processes including my nightly backup script, all of which need to access the volume mounted on my RAID array at some point. After some googling, I found that it might be down to the RAID array failing, so I checked /proc/mdstat, which shows this:
ben@jack:~$ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid5 sdb1[3](F) sdc1[1] sdd1[2]
2930271872 blocks level 5, 64k chunk, algorithm 2 [3/2] [_UU]
unused devices: <none>
And running mdadm --detail /dev/md0 gives this:
ben@jack:~$ sudo mdadm --detail /dev/md0
/dev/md0:
Version : 00.90
Creation Time : Sat Oct 31 20:53:10 2009
Raid Level : raid5
Array Size : 2930271872 (2794.53 GiB 3000.60 GB)
Used Dev Size : 1465135936 (1397.26 GiB 1500.30 GB)
Raid Devices : 3
Total Devices : 3
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Mon Mar 7 03:06:35 2011
State : active, degraded
Active Devices : 2
Working Devices : 2
Failed Devices : 1
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 64K
UUID : f114711a:c770de54:c8276759:b34deaa0
Events : 0.208245
Number Major Minor RaidDevice State
3 8 17 0 faulty spare rebuilding /dev/sdb1
1 8 33 1 active sync /dev/sdc1
2 8 49 2 active sync /dev/sdd1
I believe this says that sdb1 has failed, and so the array is running with two drives out of three 'up'. Some advice I found said to check /var/log/messages for notices of failures, and sure enough there are plenty:
ben@jack:~$ grep sdb /var/log/messages
...
Mar 7 03:06:35 jack kernel: [4525155.384937] md/raid:md0: read error NOT corrected!! (sector 400644912 on sdb1).
Mar 7 03:06:35 jack kernel: [4525155.389686] md/raid:md0: read error not correctable (sector 400644920 on sdb1).
Mar 7 03:06:35 jack kernel: [4525155.389686] md/raid:md0: read error not correctable (sector 400644928 on sdb1).
Mar 7 03:06:35 jack kernel: [4525155.389688] md/raid:md0: read error not correctable (sector 400644936 on sdb1).
Mar 7 03:06:56 jack kernel: [4525176.231603] sd 0:0:1:0: [sdb] Unhandled sense code
Mar 7 03:06:56 jack kernel: [4525176.231605] sd 0:0:1:0: [sdb] Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Mar 7 03:06:56 jack kernel: [4525176.231608] sd 0:0:1:0: [sdb] Sense Key : Medium Error [current] [descriptor]
Mar 7 03:06:56 jack kernel: [4525176.231623] sd 0:0:1:0: [sdb] Add. Sense: Unrecovered read error - auto reallocate failed
Mar 7 03:06:56 jack kernel: [4525176.231627] sd 0:0:1:0: [sdb] CDB: Read(10): 28 00 17 e1 5f bf 00 01 00 00
To me it is clear that device sdb has failed, and I need to stop the array, shutdown, replace it, reboot, then repair the array, bring it back up and mount the filesystem. I cannot hot-swap a replacement drive in, and don't want to leave the array running in a degraded state. I believe I am supposed to unmount the filesystem before stopping the array, but that is failing, and that is where I'm stuck now:
ben@jack:~$ sudo umount /storage
umount: /storage: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))
It is indeed busy; there are some 30 or 40 processes waiting on I/O.
What should I do? Should I kill all these processes and try again? Is that a wise move when they are 'uninterruptable'? What would happen if I tried to reboot?
Please let me know what you think I should do. And please ask if you need any extra information to diagnose the problem or to help!
