I've just converted a two-drive raid1 to a raid5 using "mdadm --grow --level 5 /dev/md3", and apparently this worked well:
morris:/usr/local/src/mdadm-3.1.4# cat /proc/mdstat Personalities : [raid1] [raid6] [raid5] [raid4] md3 : active raid5 sda3 sdb3 961613184 blocks level 5, 64k chunk, algorithm 2 [2/2] [uu]
Now I found I'd rather not done that, and thought I could easily revert this, but:
morris:/usr/local/src/mdadm-3.1.4# ./mdadm --grow /dev/md3 --level=1 mdadm: /dev/md3: could not set level to raid1
This is with kernel 2.6.32-bpo.2-686 on Debian 5.0.4, and mdadm 3.1.4.
I asume I could stop the raid, and simply re-assemble a new raid 1 from the two partitions, but I'm a bit reluctant to try. Is there another way to get my raid1 back?