Understanding UUID assignment

The concept is described in RFC 4122 which defines the various algorithms for generating UUIDs. Variant 4 uses random numbers and is the most common.

UUIDs are used in many scenarios, a few examples are:

• Labelling partitions and filesystems is the most common usage within Linux
• Network Manager uses them to identify network devices and connections
• Hypervisors, such as Virtual Box and Qemu-KVM, uses them to identify VMs
• The are used within Microsoft Windows where they are knows as GUIDs
• They are used for uniqueness within databases
• They are used in software development

Within Linux, all distros come with the util-linux package, which amongst other things has the uuidgen command to generate UUIDs. This is a front-end to libuuid which generates random UUIDs by default, or time-based if not enough entropy is available.

The utility you use to create a partition or filesystem will generate the UUID, which remains with that partition/filesystem until you either re-create them or explicitly change the UUID (for example with tune2fs)

UUIDs are used with filesystems and partitions in order to give them a consistent name (although a very long one). This avoids the scenario where the BIOS of UEFI firmware in a two HDD system lists hard disk so that your system disk is allocated /dev/sda on one boot and allocated /deb/sdb on the next (maybe the first disk was slower to start up on the second boot).

Using the traditional naming method, this would cause havoc in your /etc/fstab file as your system would be looking in the wrong disk for partitions to mount. For example, here is my swap entry:

/dev/sda4               none            swap            defaults        0 0

If the disks had been allocated differently at boot, my system wouldn't find a fourth partition of type swap on the non-system disk (OK, I'm on a laptop, so it hasn't got a second disk, but you get the point) and my swap would fail.

By labeling them with UUIDs, and using those within /etc/fstab you're guaranteed that the correct filesystem will be mounted at all times. For example:

UUID=d8ab8967-f2de-4c76-902f-d8d9707c399e /media/files  ext4    defaults 0 0

will always mount the partition with that UUID on /media/files regardless of the order the BIOS (or UEFI firmware) labels them at boot.