Chapter 3. Using UUIDs to Mount Devices / 3.2. Understanding UUIDs | ||||
|---|---|---|---|---|
 | Chapter 3. Using UUIDs to Mount Devices | 3.3. Using UUIDs in the Boot Loader and /etc/fstab File (x86) |  | |
Chapter 3. Using UUIDs to Mount Devices / 3.2. Understanding UUIDs | ||||
|---|---|---|---|---|
| Chapter 3. Using UUIDs to Mount Devices | 3.3. Using UUIDs in the Boot Loader and /etc/fstab File (x86) | | |
A UUID (Universally Unique Identifier) is a 128-bit number for a file system that is unique on both the local system and across other systems. It is a randomly generated with system hardware information and time stamps as part of its seed. UUIDs are commonly used to uniquely tag devices.
The UUID is always unique to the partition and does not depend on the order in which it appears or where it is mounted. With certain SAN devices attached to the server, the system partitions are renamed and moved to be the last device. For example, if root (/) is assigned to /dev/sda1 during the install, it might be assigned to /dev/sdg1 after the SAN is connected. One way to avoid this problem is to use the UUID in the boot loader and /etc/fstab files for the boot device.
A UUID never changes, no matter where the device is mounted, so it can always be found at boot. In a boot loader file, you typically specify the location of the device (such as /dev/sda1 or /dev/evms/sda1) to mount it at system boot. The boot loader can also mount devices by their UUIDs and administrator-specified volume labels. However, if you use a label and file location, you cannot change the label name when the partition is mounted.
You can use the UUID as criterion for assembling and activating software RAID devices. When a RAID is created, the md driver generates a UUID for the device, and stores the value in the md superblock.
