Persistent block device naming - ArchWiki

# Persistent block device naming

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Related articles

- fstab
- udev
- LVM

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:

- Persistent naming has limits that are out-of-scope in this article. For example, while mkinitcpio may support a method, systemd may impose its own limits (e.g. FS#42884) on naming it can process during boot.
- If you are using LVM, 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. 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.

Note: Beware that Disk cloning creates two different disks with the same name.

The following sections describes what the different persistent naming methods are and how they are used.

The lsblk 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.

    $ blkid

    /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"

### 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 -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:

swap
  `swaplabel -L <label> /dev/XXX` using util-linux
ext2/3/4
  `e2label /dev/XXX <label>` using e2fsprogs
btrfs
  `btrfs filesystem label /dev/XXX <label>` using btrfs-progs
reiserfs
  `reiserfstune -l <label> /dev/XXX` using reiserfsprogs
jfs
  `jfs_tune -L <label> /dev/XXX` using jfsutils
xfs
  `xfs_admin -L <label> /dev/XXX` using xfsprogs
fat/vfat
  `fatlabel /dev/XXX <label>` using dosfstools
  `mlabel -i /dev/XXX ::<label>` using mtools
exfat
  `exfatlabel /dev/XXX <label>` using exfat-utils
ntfs
  `ntfslabel /dev/XXX <label>` using ntfs-3g
zfs
  this filesystem does not support `/dev/disk/by-label`, but #by-partlabel may be used

Note:

- Changing the filesystem label of the root partition has to be done after booting from another partition because it needs to be unmounted first.
- Labels have to be unambiguous to prevent any possible conflicts.
- Labels can be up to 16 characters long.
- Since the label is a property of the filesystem, it is not suitable for addressing a single RAID device persistently.
- When using encrypted containers with dm-crypt, the labels of filesystems inside of containers are not available while the container is locked/encrypted.

### by-uuid

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, exFAT and NTFS filesystems 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 much less likely that name collisions occur than with labels. Further, it is generated automatically on creation of the filesystem. It will, for example, stay unique even if the device is plugged into another system (which may perhaps have a device with the same label).

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).

Tip: In case your swap partition does not have an UUID assigned, you will need to reset the swap partition using mkswap utility.

### 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. `pci-` for `by-path`, and `ata-` for `by-id`), so they are linked to the hardware controlling the device. This implies different levels of persistence: the `by-path` will already change when the device is plugged into a different port of the controller, the `by-id` will change when the device is plugged into a port of a hardware controller subject to another subsystem. [1] Thus, both are not suitable to achieve persistent naming tolerant to hardware changes.

However, both provide important information to find a particular device in a large hardware infrastructure. For example, if you do not manually assign persistent labels (`by-label` or `by-partlabel`) and keep a directory with hardware port usage, `by-id` and `by-path` can be used to find a particular device.[2] [3]

`by-id` also creates World Wide Name links of storage devices that support it. Unlike other `by-id` links, WWNs are fully persistent and will not change depending on the used subsystem.

    $ ls -l /dev/disk/by-id/

    total 0
    lrwxrwxrwx 1 root root 10 May 27 23:31 ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470 -> ../../sda
    lrwxrwxrwx 1 root root 10 May 27 23:31 ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-part1 -> ../../sda1
    lrwxrwxrwx 1 root root 10 May 27 23:31 ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-part2 -> ../../sda2
    lrwxrwxrwx 1 root root 10 May 27 23:31 ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-part3 -> ../../sda3
    lrwxrwxrwx 1 root root 10 May 27 23:31 ata-WDC_WD2500BEVT-22ZCT0_WD-WXE908VF0470-part4 -> ../../sda4
    lrwxrwxrwx 1 root root 10 May 27 23:31 wwn-0x60015ee0000b237f -> ../../sda
    lrwxrwxrwx 1 root root 10 May 27 23:31 wwn-0x60015ee0000b237f-part1 -> ../../sda1
    lrwxrwxrwx 1 root root 10 May 27 23:31 wwn-0x60015ee0000b237f-part2 -> ../../sda2
    lrwxrwxrwx 1 root root 10 May 27 23:31 wwn-0x60015ee0000b237f-part3 -> ../../sda3
    lrwxrwxrwx 1 root root 10 May 27 23:31 wwn-0x60015ee0000b237f-part4 -> ../../sda4

    $ ls -l /dev/disk/by-path/

    total 0
    lrwxrwxrwx 1 root root 10 May 27 23:31 pci-0000:00:1f.2-ata-1 -> ../../sda
    lrwxrwxrwx 1 root root 10 May 27 23:31 pci-0000:00:1f.2-ata-1-part1 -> ../../sda1
    lrwxrwxrwx 1 root root 10 May 27 23:31 pci-0000:00:1f.2-ata-1-part2 -> ../../sda2
    lrwxrwxrwx 1 root root 10 May 27 23:31 pci-0000:00:1f.2-ata-1-part3 -> ../../sda3
    lrwxrwxrwx 1 root root 10 May 27 23:31 pci-0000:00:1f.2-ata-1-part4 -> ../../sda4

### by-partlabel

Note: This method only concerns disks with GUID Partition Table (GPT).

Partition labels can be defined in the header of the partition entry on GPT disks.

See also Wikipedia:GUID Partition Table#Partition entries (LBA 2-33).

This method is very similar to the filesystem labels, excepted that the dynamic directory is `/dev/disk/by-partlabel`.

    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

Note:

- GPT partition labels also have to be different to avoid conflicts. To change your partition label, you can use gdisk or the ncurses-based version cgdisk. Both are available from the gptfdisk package. See Partitioning#Partitioning tools.
- According to the specification, GPT partition labels can be up to 72 characters long.

### by-partuuid

Note: This method only concerns disks with GUID Partition Table (GPT).

Like GPT partition labels, GPT partition UUIDs are defined in the partition entry on GPT disks. See also Wikipedia:GUID Partition Table#Partition entries.

The dynamic directory is similar to other methods and, like filesystem UUIDs, using UUIDs is preferred 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

### Static device names with Udev

See 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.

### fstab

See the main article: fstab#Identifying filesystems.

### Boot managers

To use persistent names in the boot manager (boot loader), the following prerequisites must be met. On a standard installation following the installation guide both prerequisites are met.

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

The location of the root filesystem is given by the parameter `root` on the kernel commandline. The kernel commandline is configured from the bootloader, see Kernel parameters#Configuration. To change to persistent device naming, only change the parameters which specify block devices, e.g. `root` and `resume`, while leaving other parameters as is. Various naming schemes are supported:

Persistent device naming using label and the `LABEL=` format.

    root=LABEL=System

Persistent device naming using UUID and the `UUID=` format, in this example `0a3407de-014b-458b-b5c1-848e92a327a3` is the UUID of the root partition.

    root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3

Persistent device naming using disk id and the `/dev` path format, in this example `wwn-0x60015ee0000b237f-part2` is the id of the root partition.

    root=/dev/disk/by-id/wwn-0x60015ee0000b237f-part2

Persistent device naming using GPT partition UUID and the `PARTUUID=` format.

    root=PARTUUID=98a81274-10f7-40db-872a-03df048df366

Persistent device naming using GPT partition label and the `PARTLABEL=` format.

    root=PARTLABEL=GNU\057Linux

Categories:

- Boot process
- File systems
- Hardware detection and troubleshooting