When mounting a disk, the traditional way has always been to use the name given to it by the operating system – such as hda1 (first partition on first hard drive) or sdc2 (second partition on third SCSI drive). However, with the possibility that disks may move from one location to another (such as from sdc to sda) what can be done to locate the appropriate disk to mount?
Most or all Linux systems now install with the /etc/fstab set up to use the Universally Unique Identifier (UUID). While the disk’s location could change, the UUID remains the same. How can we use the UUID?
First, you have to find out the UUID of the disk that you want to mount. There are a number of ways to do this; one traditional method has been to use the tool vol_id. This tool, however, was removed from udev back in May of 2009. There are other ways to find the UUID of a new disk.
One way is to use blkid. This command is part of util-linux-ng. The vol_id command was removed in favor of this one, and is the preferred tool to use instead of vol_id. Find the UUID of /dev/sda1 like this:
$ blkid /dev/sda1 /dev/sda1: UUID="e7b85511-58a1-45a0-9c72-72b554f01f9f" TYPE="ext3"
You could also use the tool udevadm (which takes the place of udevinfo) this way:
$ udevadm info -n /dev/sda1 -q property | grep UUID ID_FS_UUID=e7b85511-58a1-45a0-9c72-72b554f01f9f ID_FS_UUID_ENC=e7b85511-58a1-45a0-9c72-72b554f01f9f
Alternately, the tune2fs command (although specific to ext) can be used to get the UUID:
# tune2fs -l /dev/sda1 | grep UUID Filesystem UUID: e7b85511-58a1-45a0-9c72-72b554f01f9f
The tune2fs utility is also only available to the superuser, not to a normal user.
There are utilities similar to tune2fs for other filesystems – and most probably report the UUID. For instance, the XFS tool xfs_info reports the UUID (in the first line) like this:
$ xfs_info /dev/sda6
meta-data=/dev/disk/by-uuid/c68acf43-2c75-4a9d-a281-b70b5a0095e8 isize=256 agcount=4, agsize=15106816 blks
= sectsz=512 attr=2
data = bsize=4096 blocks=60427264, imaxpct=25
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0
log =internal bsize=4096 blocks=29505, version=2
= sectsz=512 sunit=0 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
The easiest way is to use the blkid command. There is yet one more way to get the UUID – and it also allows you to write programs and scripts that utilize the UUID. The filesystem contains a list of disks by their UUID in /dev/disk/by-uuid:
$ ls -l /dev/disk/by-uuid total 0 lrwxrwxrwx 1 root root 10 2011-04-01 13:08 6db9c678-7e17-4e9e-b10a-e75595c0cacb -> ../../sda5 lrwxrwxrwx 1 root root 10 2011-04-01 13:08 c68acf43-2c75-4a9d-a281-b70b5a0095e8 -> ../../sda6 lrwxrwxrwx 1 root root 10 2011-04-01 13:08 e7b85511-58a1-45a0-9c72-72b554f01f9f -> ../../sda1
Since a disk drive is just a file, these links can be used to “find” a disk device by UUID – and to identify the UUID as well. Just use /dev/disk/by-uuid/e7b85511-58a1-45a0-9c72-72b554f01f9f in your scripts instead of /dev/sda1 and you’ll be able to locate the disk no matter where it is (as long as /dev/disk/by-uuid exists and works).
For example, let’s say you want to do a disk image copy of what is now /dev/sda1 – but you want the script to be portable and to find the disk no matter where it winds up. Using dd and gzip, you can do something like this:
UUID=e7b85511-58a1-45a0-9c72-72b554f01f9f
dd if=/dev/disk/by-uuid/${UUID} of=- | gzip -c 9 > backup-$UUID.gz
You can script the retrieval of the UUID – perhaps from a user – this way:
eval `blkid -o udev ${DEV}`
echo "UUID of ${DEV} is ${ID_FS_UUID}"
In the /etc/fstab file, replace the mount device (such as /dev/sda1) with the phrase UUID=e7b85511-58a1-45a0-9c72-72b554f01f9f (or whatever the UUID actually is).
Be aware that the UUID is associated with the filesystem, not the device itself – so if you reformat the device with a new filesystem (of whatever type) the UUID will change.
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