Using UUID to Deal With Disks

Wednesday Dec 17th 2008 by Juliet Kemp

Tip of the Trade: UUID notation is one way to access your disks when using Linux. But first you must find the identifier.

Juliet Kemp
UUID (Universal Unique Identifier) notation is another way provided by Linux to access your disks. Unlike the standard /dev/hda1, /dev/sda2 notation, it is linked specifically to a particular piece of hardware (whereas /dev/ notation can shift between disks depending on how they're plugged in). This means you don't have the risk of a system reconfiguration causing disks to be renamed, and thus causing knock-on breakage of things that reference that disk.

OK, so, first, how do you find out the UUID of your disk? The command blkid can be used — here's its output for one of my disks:

# blkid /dev/hda1
/dev/hda1: UUID="0ef96300-36c0-4575-8a1d-2d36ff4cd585" SEC_TYPE="ext2" TYPE="ext3"
Alternatively, you can look at the /dev/disk/by_uuid/ directory to see which UUID is associated with which disk. (Use df to find out what device names you currently have mounted.)

You can also do this the other way around, starting from the UUID: findfs UUID=uuid will tell you which disk is associated with that UUID.

Once you have the UUID of a device, you can use it with mount by using the -U switch:

mount -U 0ef96300-36c0-4575-8a1d-2d36ff4cd585 /mnt/disk1
or, you can use it in your /etc/fstab by putting UUID=uuid at the start of the relevant line, instead of /dev/hdaX.

To assign a UUID to a disk, use tunefs. You can generate your own UUID with the command uuidgen. Then type:

tune2fs -U uuid /dev/sda2
This might come in handy if, say, you've cloned your disk (e.g., by using dd) and must be able to mount both the original and the clone. Assign it a new UUID and you're away!

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