Penguin
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mdadm: device is too small: 0K

mdadm: /dev/sda3 is too small: 0K
mdadm: create aborted

Check that the /dev/sda3 special file is actually correct. If you inadvertantly screw up your mdadm command (eg giving /dev/sda3 as the first non-option argument), then mdadm will quite happily replace your /dev/sda3 special with an md special:

# ls -ld /dev/sda*
brw-rw---- 1 root disk 8, 0 2009-05-05 18:20 /dev/sda
brw-rw---- 1 root disk 8, 1 2009-05-05 19:14 /dev/sda1
brw-rw---- 1 root disk 8, 2 2009-05-05 19:19 /dev/sda2
brw-r--r-- 1 root root 9, 0 2009-05-05 20:06 /dev/sda3

To fix (obviously replacing "sda3" with whatever device you are actually having problems with):

# rm /dev/sda3
# mknod /dev/sda3 b 8 3

Migrating an existing partition to raid1

You can create a raid1 that is initially degraded; once all the data is on it and you are running from that, then you can add more devices to the raid1 and it will "rebuild" it in the background.

Eg:

# mdadm --create /dev/md0 --raid-devices=2 --level=raid1 /dev/sdb1 missing

  size=296696452K  mtime=Tue May  5 19:29:00 2009
  mdadm: array /dev/md0 started.
# (mkfs.xxx /dev/md0)
# (mount and cp stuff from sda1 to /dev/md0)
# ... (once sda1 is no longer in use, and the same size)
# mdadm --add /dev/md0 /dev/sda1

Resources:

CategoryDiskNotes