mdadm: assembling another array using device removed from active one

Question

In an attempt to enlarge raid1 array on a working machine, I am testing following scenario:

• remove one of two disks
• add bigger disk, resync
• add another bigger disk, resync, grow to raid-devices=3
• "fail" and remove the original, small disk
• "grow" the device back to raid-devices=2
• make a backup raid device using the small disk
• enlarge the main device (now containing two big disks)

Now everything seems to work except one important thing: I am unable to assemble a new md device using the disk "failed" and removed from the original md10 device:

mdadm --assemble /dev/md11 /dev/sde3
mdadm: Found some drive for an array that is already active: /dev/md10
mdadm: giving up

Is there any way to somehow change the identification of the array on such a removed device?

Answer 1

mdadm is complaining that you're trying to assemble an array that has a UUID matching one that is already running. You'll need to change the UUID on the /dev/sde3 superblock as you assemble it using the --update=uuid and --uuid=<newuuid> parameters. This should tell mdadm to use the UUID you're passing in when comparing to the running arrays.

To find the old UUID, use mdadm --examine /dev/sde3 and grab the old UUID. You could change it to be unique, or generate a new one.

Your new command would be: mdadm --assemble /dev/md11 --update=uuid --uuid=<newuuid> /dev/sde3