Persistent block device naming

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This article describes how to use persistent names for your block devices. This has been made possible by the introduction of udev and has some advantages over bus-based naming. If your machine has more than one SATA, SCSI or IDE disk controller, the order in which their corresponding device nodes are added is arbitrary. This may result in device names like /dev/sda and /dev/sdb switching around on each boot, culminating in an unbootable system, kernel panic, or a block device disappearing. Persistent naming solves these issues.

Note: If you are using LVM2, this article is not relevant as LVM takes care of this automatically.

Persistent naming methods

There are four different schemes for persistent naming: by-label, by-uuid, by-id and by-path. The following sections describes what the different persistent naming methods are and how they are used.

Here is a good command for viewing all the information.

# blkid -o list -c /dev/null
device         fs_type  label     mount point        UUID
------------------------------------------------------------------------------------------
/dev/sda1      ext2               /boot              7f4cef7e-7ee2-489a-b759-d52ba23b692c
/dev/sda2      swap               (not mounted)      a807fff3-e89f-46d0-ab17-9b7ad3efa7b5
/dev/sda3      ext4               /                  81917291-fd1a-4ffe-b95f-61c05cfba76f
/dev/sda4      ext4               /home              c4c23598-19fb-4562-892b-6fb18a09c7d3
/dev/sdb1      ext4     X2        /mnt/X1            4bf265f7-da17-4575-8758-acd40885617b
/dev/sdc1      ext4     X1        /mnt/X2            4bf265f7-da17-4575-8758-acd40885617b
/dev/sdd1      ext4     Y2        /mnt/Y2            8a976a06-3e56-476f-b73a-ea3cad41d915
/dev/sde1      ext4     Z2        /mnt/Z2            9d35eaae-983f-4eba-abc9-434ecd4da09c
/dev/sdf1      ext4     Y1        /mnt/Y1            e2ec37a9-0689-46a8-a07b-0609ce2b7ea2
/dev/sdg1      ext4     Z1        /mnt/Z1            9fa239c1-720f-42e0-8aed-39cf53a743ed
/dev/sdj1      ext4     RAPT      (not mounted)      a9ed7ecb-96ce-40fe-92fa-e07a532ed157
/dev/sdj2      swap               <swap>             20826c74-eb6d-46f8-84d8-69b933a4bf3f

by-label

Almost every filesystem type can have a label. All your partitions that have one are listed in the /dev/disk/by-label directory. This directory is created and destroyed dynamically, depending on whether you have partitions with labels attached.

$ ls -lF /dev/disk/by-label
total 0
lrwxrwxrwx 1 root root 10 Oct 16 10:27 data -> ../../sdb2
lrwxrwxrwx 1 root root 10 Oct 16 10:27 data2 -> ../../sda2
lrwxrwxrwx 1 root root 10 Oct 16 10:27 fat -> ../../sda6
lrwxrwxrwx 1 root root 10 Oct 16 10:27 home -> ../../sda7
lrwxrwxrwx 1 root root 10 Oct 16 10:27 root -> ../../sda1
lrwxrwxrwx 1 root root 10 Oct 16 10:27 swap -> ../../sda5
lrwxrwxrwx 1 root root 10 Oct 16 10:27 windows -> ../../sdb1

The labels of your filesystems can be changed, but note that they have to be unambiguous to prevent any possible conflicts. Following are some methods for changing labels on common filesystems:

swap 
# swaplabel -L <label> /dev/XXX
ext2/3/4 
# e2label /dev/XXX <label>
btrfs 
# btrfs filesystem label /dev/XXX <label>
reiserfs 
# reiserfstune -l <label> /dev/XXX
jfs 
# jfs_tune -L <label> /dev/XXX
xfs 
# xfs_admin -L <label> /dev/XXX
fat/vfat
# dosfslabel /dev/XXX <label>  # dosfstools package
# mlabel -i /dev/XXX ::<label>  # mtools package; converts label to uppercase
ntfs (ntfsprogs package) 
# ntfslabel /dev/XXX <label>
Note: Labels can be up to 16 characters long.

by-uuid

UUID is a mechanism to give each filesystem a unique identifier. It is designed so that collisions are unlikely. All GNU/Linux filesystems (including swap and LUKS headers of raw encrypted devices) support UUID. FAT and NTFS filesystems (fat and windows labels above) do not support UUID, but are still listed in /dev/disk/by-uuid with a shorter UID (unique identifier):

# ls -l /dev/disk/by-uuid/
total 0
lrwxrwxrwx 1 root root 10 Oct 16 10:27 2d781b26-0285-421a-b9d0-d4a0d3b55680 -> ../../sda1
lrwxrwxrwx 1 root root 10 Oct 16 10:27 31f8eb0d-612b-4805-835e-0e6d8b8c5591 -> ../../sda7
lrwxrwxrwx 1 root root 10 Oct 16 10:27 3FC2-3DDB -> ../../sda6
lrwxrwxrwx 1 root root 10 Oct 16 10:27 5090093f-e023-4a93-b2b6-8a9568dd23dc -> ../../sda2
lrwxrwxrwx 1 root root 10 Oct 16 10:27 912c7844-5430-4eea-b55c-e23f8959a8ee -> ../../sda5
lrwxrwxrwx 1 root root 10 Oct 16 10:27 B0DC1977DC193954 -> ../../sdb1
lrwxrwxrwx 1 root root 10 Oct 16 10:27 bae98338-ec29-4beb-aacf-107e44599b2e -> ../../sdb2

The advantage about using the UUID method is that it is less likely that you have name collisions than with labels. The disadvantage is that UUIDs make long code lines hard to read and break formatting in many configuration files (e.g. fstab or crypttab). Also every time a partition is resized or reformatted a new UUID is generated and configs have to get adjusted (manually).

by-id and by-path

by-id creates a unique name depending on the hardware serial number, by-path depending on the shortest physical path (according to sysfs). Both contain strings to indicate which subsystem they belong to (i.e. -ide- for by-path, and -ata- for by-id) and thus are not suitable for solving the problems mentioned in the beginning of this article. They will not be discussed any further here.

Individual device names

You can also create individual device names: udev#Setting static device names (for iscsi).

Using persistent naming

There are various applications that can be configured using persistent naming. Following are some examples of how to configure them.

fstab

To enable persistent naming in /etc/fstab replace the device kernel name in the first column with the persistent name path as follows:

/dev/disk/by-label/home_myhost ...
/dev/disk/by-uuid/31f8eb0d-612b-4805-835e-0e6d8b8c5591 [...]

or directly specify the persistent name type using a prefix:

LABEL=home_myhost ...
UUID=1f8eb0d-612b-4805-835e-0e6d8b8c5591 [...]

Boot managers

To use persistent names in your boot manager, the following prerequisites must be met:

  • You are using a mkinitcpio initial RAM disk image
  • You have udev enabled in /etc/mkinitcpio.conf

In the above example, /dev/sda1 is the root partition. In the GRUB2 grub.cfg file, the linux line looks like this:

linux /boot/vmlinuz-linux root=/dev/sda1 ro quiet

Depending on which naming scheme you would prefer, change it to one of the following:

linux /boot/vmlinuz-linux root=/dev/disk/by-label/root_myhost ro quiet

or:

linux /boot/vmlinuz-linux root=UUID=2d781b26-0285-421a-b9d0-d4a0d3b55680 ro quiet

If you are using LILO, then do not try this with the root=... configuration option; it will not work. Use append="root=..." or addappend="root=..." instead. Read the LILO man page for more information on append and addappend.

There is an alternative way to use the label embedded in the filesystem. For example if (as above) the filesystem in /dev/sda1 is labeled root_myhost, you would give this line to GRUB2:

linux /boot/vmlinuz-linux root=LABEL=root_myhost ro quiet