Comment 1 for bug 252365

I can confirm this problem on Ubuntu Server 8.04. I have the same RAID5 configuration as the reporter (4 active disks, 1 spare).

In my case the spare isn't added to the array on boot no matter what changes are made to mdadm.conf. Stating the devices explicitly doesn't help.

$ cat /etc/mdadm/mdadm.conf
[snip]
DEVICE partitions
[snip]
ARRAY /dev/md0 level=raid5 num-devices=4 spares=1 UUID=d7ae6761:1abc675c:6e03419f:9b214e6e

$ sudo mdadm --detail --scan
ARRAY /dev/md0 level=raid5 num-devices=4 spares=1 UUID=d7ae6761:1abc675c:6e03419f:9b214e6e

$ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid5 sdf1[4](S) sda1[0] sdd1[3] sdc1[2] sdb1[1]
      2197715712 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]

unused devices: <none>

$ sudo mdadm --detail /dev/md0
/dev/md0:
        Version : 00.90.03
  Creation Time : Thu Aug 9 15:29:00 2007
     Raid Level : raid5
     Array Size : 2197715712 (2095.91 GiB 2250.46 GB)
  Used Dev Size : 732571904 (698.64 GiB 750.15 GB)
   Raid Devices : 4
  Total Devices : 5
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Fri Nov 28 15:06:38 2008
          State : clean
 Active Devices : 4
Working Devices : 5
 Failed Devices : 0
  Spare Devices : 1

         Layout : left-symmetric
     Chunk Size : 64K

           UUID : d7ae6761:1abc675c:6e03419f:9b214e6e
         Events : 0.76

    Number Major Minor RaidDevice State
       0 8 1 0 active sync /dev/sda1
       1 8 17 1 active sync /dev/sdb1
       2 8 33 2 active sync /dev/sdc1
       3 8 49 3 active sync /dev/sdd1

       4 8 81 - spare /dev/sdf1