Add a spare to a poorly defined raid1

Not knowing what I was doing, I created a RAID1 with a single drive and a spare, as below.

I'd like to add the spare to the array. How do I do this without rebooting?

Cheers,
Dave

# definitions of existing MD arrays
ARRAY /dev/md0 level=raid1 num-devices=1 UUID=9921faab:5d93510e:0db287f1:dc103b07
   spares=1

# mdadm --detail /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Fri Jul 18 14:57:40 2008
     Raid Level : raid1
     Array Size : 104320 (101.89 MiB 106.82 MB)
  Used Dev Size : 104320 (101.89 MiB 106.82 MB)
   Raid Devices : 1
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

Update Time : Wed Feb 15 03:37:11 2012
State : clean
Active Devices : 1
Working Devices : 2
Failed Devices : 0
Spare Devices : 1

UUID : 9921faab:5d93510e:0db287f1:dc103b07
Events : 0.218

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

1 8 17 - spare /dev/sdb1

That's a straightforward one - you just need to increase the number of active members in the array:

    mdadm --grow /dev/md0 -n 2

Cheers,
Robin

     ___        
    ( ' }     |       Robin Hill        <[email protected]> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

Perfect, thank you!