The mystical UUID

Every filesystem (partition?) should have a uuid. On modern Linux systems you can see them with the following command:

# ls -l /dev/disk/by-uuid/
total 0
lrwxrwxrwx 1 root root 10 2008-01-25 19:44 c2ceffb9-90be-4564-a946-9d37de7725ba -> ../../hdg2

UUIDs, while a bit cumbersome to look at, are extremely nice because you can use them in a lot of places instead of a normal device name (such as /dev/hdg2 in the above example).

Tonight I moved the 2 hard drives I had plugged into the onboard IDE controller of my motherboard into a Promise Ultra100 card. Because of this, the kernel renamed the partitions from /dev/hda and /dev/hdc to /dev/hde/ and /dev/hdg. Upon booting the system I saw the following:

swapon: cannot canonicalize /dev/hda2: No such file or directory
swapon: cannot stat /dev/hda2: No such file or directory
swapon: cannot canonicalize /dev/hdc2: No such file or directory
swapon: cannot stat /dev/hdc2: No such file or directory

UUIDs will help this to never happen again.

Here are the relevant lines from my old /etc/fstab:

/dev/hda2   none    swap    sw,pri=1    0 0
/dev/hdc2   none    swap    sw,pri=1    0 0

And the new lines:

UUID=fe6bffd9-5b6b-4db9-8929-cf1575a72d67   none    swap    sw,pri=1    0 0
UUID=e2992cf5-bc3a-4b3a-a920-d9dfbe7a5a9a   none    swap    sw,pri=1    0 0

As I said, it doesn't look as pretty, but look what happens with the old /etc/fstab:

#swapon -a
# cat /proc/swaps
Filename                                Type            Size    Used    Priority

and the new:

# swapon -a
erma ~ # cat /proc/swaps
Filename                                Type            Size    Used    Priority
/dev/hde2                               partition       1004052 0       1
/dev/hdg2                               partition       1004052 0       1

If you haven't figured it out by now, by specifying partitions by UUID, you remove the dependency on where they are physically plugged into the motherboard and any kernel naming conventions. I recently had my SATA drives move around a bit after a BIOS update, so UUIDs would help out there as well.

As it happens I had some trouble finding the (correct) UUID of one of my swap partitions but that's the topic of my next post.