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!