Persistent block device naming
|Summary help replacing me|
|An overview of persistent block device naming; the preferred method of referencing block devices.|
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/sdb switching around on each boot, culminating in an unbootable system, kernel panic, or a block device disappearing. Persistent naming solves these issues.
Persistent naming methods
Here is a good command for viewing all the information:
$ lsblk -f
NAME FSTYPE LABEL UUID MOUNTPOINT sda ├─sda1 ext2 7f4cef7e-7ee2-489a-b759-d52ba23b692c /boot ├─sda2 swap a807fff3-e89f-46d0-ab17-9b7ad3efa7b5 [SWAP] ├─sda3 ext4 81917291-fd1a-4ffe-b95f-61c05cfba76f / └─sda4 ext4 c4c23598-19fb-4562-892b-6fb18a09c7d3 /home sdb └─sdb1 ext4 X2 4bf265f7-da17-4575-8758-acd40885617b /mnt/X1 sdc └─sdc1 ext4 X1 4bf265f7-da17-4575-8758-acd40885617b /mnt/X2 sdd └─sdd1 ext4 Y2 8a976a06-3e56-476f-b73a-ea3cad41d915 /mnt/Y2 sde └─sde1 ext4 Z2 9d35eaae-983f-4eba-abc9-434ecd4da09c /mnt/Z2 sdf └─sdf1 ext4 Y1 e2ec37a9-0689-46a8-a07b-0609ce2b7ea2 /mnt/Y1 sdg └─sdg1 ext4 Z1 9fa239c1-720f-42e0-8aed-39cf53a743ed /mnt/Z1 sdj ├─sdj1 ext4 RAPT a9ed7ecb-96ce-40fe-92fa-e07a532ed157 └─sdj2 swap 20826c74-eb6d-46f8-84d8-69b933a4bf3f [SWAP]
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 -l /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:
swaplabel -L <label> /dev/XXXusing
e2label /dev/XXX <label>using
btrfs filesystem label /dev/XXX <label>using
reiserfstune -l <label> /dev/XXXusing
jfs_tune -L <label> /dev/XXXusing
xfs_admin -L <label> /dev/XXXusing
dosfslabel /dev/XXX <label>using
mlabel -i /dev/XXX ::<label>using
ntfslabel /dev/XXX <label>using
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.
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.
Using persistent naming
There are various applications that can be configured using persistent naming. Following are some examples of how to configure them.
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 [...]
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
In the above example,
/dev/sda1 is the root partition. In the GRUB
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
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
addappend="root=..." instead. Read the LILO man page for more information on
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 GRUB:
linux /boot/vmlinuz-linux root=LABEL=root_myhost ro quiet