3 Using UUIDs to Mount Devices #
This section describes the use of UUIDs (Universally Unique Identifiers)
instead of device names (such as /dev/sda1
) to identify
file system devices. Starting with SUSE Linux Enterprise Server 12, UUIDs are used by
default in the boot loader file and the /etc/fstab
file.
3.1 Persistent Device Names with udev #
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.
Other programs besides udev
, such as LVM or
md
, might also generate UUIDs, but they are not listed
in /dev/disk
.
3.2 Understanding UUIDs #
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.
One way to avoid this problem is to use the UUID in the boot loader and
/etc/fstab
files for the boot device. This is the
default in SUSE Linux Enterprise since version 12. 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:
tux >
ls -og /dev/disk/by-uuid/
lrwxrwxrwx 1 10 Dec 5 07:48 e014e482-1c2d-4d09-84ec-61b3aefde77a -> ../../sda1
3.3 Additional Information #
For more information about using udev
for managing
devices, see Chapter 22, Dynamic Kernel Device Management with udev
.
For more information about udev
commands, see
man 7 udev
.
3.4 Mounting network storage devices #
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
option to the /etc/fstab
file for each particular network storage device. An example follows:
mars.example.org:/nfsexport /shared nfs defaults,_netdev 0 0