Migrate mdadm raid array from 32bit to 64bit debian squeeze

Question

I have a raid5 array running on Debian Squeeze 32bit (x86) and I plan to upgrade to Debian Squeeze 64bit (amd64). To keep my raid5 array and ext4 filesystem I simply keep a copy of my mdadm.conf file. I am pretty confident this is enough to get my array up and running again once I am on 64bit, but I am still a bit nervous in case there is something I have forgotten (and shhhhh I know I am missing the backup!).

I plan to also upgrade the raid5 array to a raid6 array afterwards.

1. Is there a problem migrating a raid5 created on a 32bit to 64bit system?
2. Do I need more than mdadm.conf ?
3. Is there anything else I need to know?

Answer 1

As a bit of comfort, I'm pretty sure mdadm won't mount the array if it's not 100% sure it can. Worst case, just re-install 32-bit to recover the array.

Technically, I don't even think you need mdadm.conf, as long as you know which devices are in the array and what level it is (I've manually moved a raid5 array from one system to another without issue. mdadm writes a good bit of the raid information on the superblock of each disk, so you should be able to bring the array up from a new install or even a live cd with mdadm --assemble /dev/disk1 /dev/disk2 /dev/disk3 ...). However, I've not tested moving between architectures (32bit -> 64bit) before.

The only other notes I'd mention are don't try to bring it up with only 4 devices, as this will update the superblock on those 4 devices, resulting in the 5th device appearing as out-of-date (even if you immediately stop the array), which will cause you to have to fail the 5th device out of the array and re-add it and let it rebuild. This issue can generally be avoided by the following advice:

Don't use --force unless you are absolutely sure it's what you want to do. There is no --unforce.

Answer 2

I've now done it and the answer to my own questions are:

1. No
2. You don't even need mdadm.conf
3. It is smart with a backup.