Can you change the metadata version on an existing array?

Question

So grub requires metadata version 0.90 I'm wondering if I can change my existing arrays to use that instead of the default which is 1.2. That way I don't have to go about a reinstall. If yes, how can I do it?

Answer 1

The GRUB wiki confirms:

Also, (as of 1.96+20080724) GRUB can only boot from RAID which uses a Version 0.90 metadata superblock (i.e. one created with the --metadata=0.90 option to mdadm).

Curiously, the man page for mdadm says

-e , --metadata=

Declare the style of superblock (raid metadata) to be used. The default is 0.90 for --create, and to guess for other operations.

I'd double check with mdadm -Q -D <device> to be certain.

After that, What RAID level are you running? The best scenario I can think of for a mirror (RAID 1) would be

1. Take backups
2. Unmount array 0
3. Remove device A from array 0 of n devices
4. Zero superblock on device A
5. Create array 1 using device A and n-1 spares, explicitly using old superblock format
6. Repeat for remaining B through n-1 devices
7. (Optional) re-number the array

... but very definitely check that you absolutely need to go through with this!

Answer 2

This information is probably to late to help the op, but maybe it will help someone else. The first command wipes the super blocks and the second command creates a new array but assumes the devices are clean. I have just performed this and everything appears to be ok. I would definitely recommend a backup before doing this. I had no data to loose and therefore no integrity to check. I just wanted to avoid another 4 hour resync.

mdadm --zero-superblock /dev/sd[a-z]1

mdadm --create /dev/md0 --assume-clean --level=10 --raid-devices=10 /dev/sd[a-z]1 --metadata=0.90