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I have a RAID array that I cannot seem to mount after a power failure. The mount error says it cannot find the UUID even though it is to UUID reported by mdadm.

mdadm --examine --scan

ARRAY /dev/md/0  metadata=1.2 UUID=4a9ed4ce:505da073:afd780ed:3e5d5622 name=nas:0

fstab entry is:

UUID=4a9ed4ce:505da073:afd780ed:3e5d5622 /md0 ext4 defaults 0 0

Trying to mount the device:

mount /md0

mount: /md0: can't find UUID=4a9ed4ce:505da073:afd780ed:3e5d5622.

My mdadm.conf:

ARRAY /dev/md/nas:0 level=raid5 num-devices=6 metadata=1.2 name=nas:0 UUID=4a9ed4ce:505da073:afd780ed:3e5d5622
   devices=/dev/sda1,/dev/sdb1,/dev/sdc1,/dev/sdd1,/dev/sde1,/dev/sdf1

Can someone help me figure this out? Thanks.

As an aside, I don't know if this is a Ubuntu thing but I find it odd the device is /dev/md/0 rather than /dev/md0.

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  • I'm not sure what this means but I was able to mount the RAID using /dev/md/nas:0 rather than the UUID. Commented Aug 24, 2023 at 16:21

1 Answer 1

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mdadm looks for a RAID set UUID, while mount looks for a filesystem UUID. Unless someone or something specifically assigned them to the same value at filesystem creation time, there is no reason the filesystem UUID should be the same as the RAID set UUID.

It appears the RAID set UUID is correctly configured in mdadm.conf, but the filesystem UUID in /etc/fstab might be incorrect. See lsblk -o +uuid or sudo blkid, once the RAID set is successfully started.

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  • Thanks for the reply but that just adds to my confusion. I know the UUID in fstab is wrong but finding the right one is where I'm stuck. Both commands list the UUID of the individual drives that are members of the RAID. How do I know which is the UUID of the file system? Is there anything wrong with using /dev/md/nas:0 instead? It seems to work. Commented Aug 25, 2023 at 14:42
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    Well, another way to get the UUID of a specific ext4 filesystem that applies to this situation would be tune2fs -l /dev/md/nas:0 | grep UUID. Since /dev/md/nas:0 uses an array name which is (hopefully) specific to this array, you could just use that instead of an UUID, if you feel it's more convenient to you. Commented Aug 25, 2023 at 15:37
  • Thanks. Using /dev is easier as long as it is safe. That was what I was wondering. Commented Aug 26, 2023 at 16:12

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