3 Mounting storage devices #

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 randomly generated with system hardware information and time stamps as part of its seed. UUIDs are commonly used to uniquely tag devices.

Using non-persistent “traditional” device names such as /dev/sda1 may render the system unbootable when adding storage. For example, if root (/) is assigned to /dev/sda1, it might be reassigned to /dev/sdg1 after a SAN has been attached or additional hard disks have been applied to the system. In this case the boot loader configuration and the /etc/fstab file need to be adjusted, otherwise the system will no longer boot.

By default UUID are used in the boot loader and /etc/fstab files for the boot device. The UUID is a property of the file system and can change if you reformat the drive. Other alternatives to using UUIDs of device names would be to identify devices by ID or label.

You can also 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.

You can find the UUID for any block device in the /dev/disk/by-uuid directory. For example, a UUID entry looks like this:

> ls -og /dev/disk/by-uuid/
lrwxrwxrwx 1 10 Dec  5 07:48 e014e482-1c2d-4d09-84ec-61b3aefde77a -> ../../sda1

Starting with Linux kernel 2.6, udev provides a user space solution for the dynamic /dev directory, with persistent device naming. As part of the hotplug system, udev is executed if a device is added to or removed from the system.

A list of rules is used to match against specific device attributes. The udev rules infrastructure (defined in the /etc/udev/rules.d directory) provides stable names for all disk devices, regardless of their order of recognition or the connection used for the device. The udev tools examine every appropriate block device that the kernel creates to apply naming rules based on certain buses, drive types, or file systems. For information about how to define your own rules for udev, see Writing udev Rules.

Along with the dynamic kernel-provided device node name, udev maintains classes of persistent symbolic links pointing to the device in the /dev/disk directory, which is further categorized by the by-id, by-label, by-path, and by-uuid subdirectories.

Note: UUID generators

Other programs besides udev, such as LVM or md, might also generate UUIDs, but they are not listed in /dev/disk.

For more information about using udev for managing devices, see Chapter 24, Dynamic Kernel Device Management with udev.

For more information about udev commands, see man 7 udev.

Some types of storage devices require network to be configured and available before systemd.mount starts to mount the devices. To postpone mounting of these types of devices, add the _netdev and x-systemd.requires=iscsi.service options to the /etc/fstab file for each particular network storage device. An example follows:

mars.example.org:/nfsexport  /shared   nfs  defaults,_netdev,x-systemd.requires=iscsi.service    0  0

Do not use the nofail option as booting of the machine continues without waiting for the particular storage device to be successfully mounted.