Difference between revisions of "Persistent block device naming"
(→Static device names with Udev: Udev#Setting_static_device_names has relevant subsections, see https://wiki.archlinux.org/index.php?title=Persistent_block_device_naming&diff=prev&oldid=186846 , ?title=Udev&diff=186842&oldid=186750 + related)
(Deleted: →Static device names with Udev: . See https://wiki.archlinux.org/index.php/Talk:Udev#Removed_.2F.2A_iscsi_device_.2A.2F_:_Wrong_info.)
|Line 120:||Line 120:|
==Using persistent naming==
==Using persistent naming==
Revision as of 08:57, 8 October 2014
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
There are four different schemes for persistent naming: by-label, by-uuid, by-id and by-path. For those using disks with GUID Partition Table (GPT), two additional schemes can be used by-partlabel and by-partuuid. You can also use static device names by using Udev.
The following sections describes what the different persistent naming methods are and how they are used.
lsblk -f command can be used for viewing graphically the first persistent schemes:
$ lsblk -f
NAME FSTYPE LABEL UUID MOUNTPOINT sda ├─sda1 vfat CBB6-24F2 /boot ├─sda2 ext4 SYSTEM 0a3407de-014b-458b-b5c1-848e92a327a3 / ├─sda3 ext4 DATA b411dc99-f0a0-4c87-9e05-184977be8539 /home └─sda4 swap f9fe0b69-a280-415d-a03a-a32752370dee [SWAP]
For those using GPT, use the
blkid command instead. The latter is more convenient for scripts, but more difficult to read.
/dev/sda1: UUID="CBB6-24F2" TYPE="vfat" PARTLABEL="EFI SYSTEM PARTITION" PARTUUID="d0d0d110-0a71-4ed6-936a-304969ea36af" /dev/sda2: LABEL="SYSTEM" UUID="0a3407de-014b-458b-b5c1-848e92a327a3" TYPE="ext4" PARTLABEL="GNU/LINUX" PARTUUID="98a81274-10f7-40db-872a-03df048df366" /dev/sda3: LABEL="DATA" UUID="b411dc99-f0a0-4c87-9e05-184977be8539" TYPE="ext4" PARTLABEL="HOME" PARTUUID="7280201c-fc5d-40f2-a9b2-466611d3d49e" /dev/sda4: UUID="f9fe0b69-a280-415d-a03a-a32752370dee" TYPE="swap" PARTLABEL="SWAP" PARTUUID="039b6c1c-7553-4455-9537-1befbc9fbc5b"
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 May 27 23:31 DATA -> ../../sda3 lrwxrwxrwx 1 root root 10 May 27 23:31 SYSTEM -> ../../sda2
The labels of your filesystems can be changed. 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
- Changing the filesystem label of the root partition has to be done from a "live" GNU/Linux distribution because the partition needs to be unmounted first.
- Labels have to be unambiguous to prevent any possible conflicts;
- Labels can be up to 16 characters long.
UUID is a mechanism to give each filesystem a unique identifier. These identifiers are generated by filesystem utilities (e.g.
mkfs.*) when the partition gets formatted and are 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 May 27 23:31 0a3407de-014b-458b-b5c1-848e92a327a3 -> ../../sda2 lrwxrwxrwx 1 root root 10 May 27 23:31 b411dc99-f0a0-4c87-9e05-184977be8539 -> ../../sda3 lrwxrwxrwx 1 root root 10 May 27 23:31 CBB6-24F2 -> ../../sda1 lrwxrwxrwx 1 root root 10 May 27 23:31 f9fe0b69-a280-415d-a03a-a32752370dee -> ../../sda4
The advantage of 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.
Partition labels can be defined in the header of the partition entry on GPT disks. Template:Wikipedia
This method is very similar to the filesystem labels, excepted that the dynamic directory is
ls -l /dev/disk/by-partlabel/
total 0 lrwxrwxrwx 1 root root 10 May 27 23:31 EFI\x20SYSTEM\x20PARTITION -> ../../sda1 lrwxrwxrwx 1 root root 10 May 27 23:31 GNU\x2fLINUX -> ../../sda2 lrwxrwxrwx 1 root root 10 May 27 23:31 HOME -> ../../sda3 lrwxrwxrwx 1 root root 10 May 27 23:31 SWAP -> ../../sda4
- GPT partition labels have also to be different to avoid conflicts. To change your partition label, you can use
gdiskor the ncurse-based version
cgdisk. Both are available from the package. See Partitioning#Partitioning_tools.
- According to the specification, GPT partition labels can be up to 72 characters long.
The dynamic directory is similar to other methods and, like UUID filesystems, using UUIDs is prefered over labels.
ls -l /dev/disk/by-partuuid/
total 0 lrwxrwxrwx 1 root root 10 May 27 23:31 039b6c1c-7553-4455-9537-1befbc9fbc5b -> ../../sda4 lrwxrwxrwx 1 root root 10 May 27 23:31 7280201c-fc5d-40f2-a9b2-466611d3d49e -> ../../sda3 lrwxrwxrwx 1 root root 10 May 27 23:31 98a81274-10f7-40db-872a-03df048df366 -> ../../sda2 lrwxrwxrwx 1 root root 10 May 27 23:31 d0d0d110-0a71-4ed6-936a-304969ea36af -> ../../sda1
Using persistent naming
There are various applications that can be configured using persistent naming. Following are some examples of how to configure them.
See the main article: fstab#UUID
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 rw 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 rw quiet
linux /boot/vmlinuz-linux root=UUID=2d781b26-0285-421a-b9d0-d4a0d3b55680 rw 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 rw quiet