永続的なブロックデバイスの命名

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この記事ではブロックデバイスに永続的な名前を使う法法を説明します。永続的な命名は udev によって可能になったものであり、バスによる命名と比べて複数の利点があります。使用しているマシンに複数の SATA, SCSI, IDE ディスクコントローラが存在する場合、それぞれのデバイスノードが追加される順番は一定しません。そのため、起動するたびに /dev/sda や /dev/sdb といったデバイス名は入れ替わる可能性があり、下手をすると、システムが起動できなくなったり、カーネルパニックが発生したり、ブロックデバイスが表示されなくなってしまいます。永続的な命名によってこれらの問題は解決します。

ノート:
  • 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 LVM2, this article is not relevant as LVM takes care of this automatically.

永続的な命名の方法

永続的な命名には4つの異なる形式があります: by-label, by-uuid, by-id と by-path。GUID Partition Table (GPT) のディスクを使用する場合、さらに by-partlabel と by-partuuid の形式も利用可能です。また、Udev による固定デバイス名を使うこともできます。

以下のセクションでは4つの異なる命名規則がどのようなものであるか、またどうやって使うのかを説明しています。

The 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. 

$ 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 
dosfslabel /dev/XXX <label> using dosfstools
fat/vfat 
mlabel -i /dev/XXX ::<label> using mtools
ntfs 
ntfslabel /dev/XXX <label> using ntfs-3g
zfs 
this filesystem does not support /dev/disk/by-label, but #by-partlabel may be used
ノート:
  • 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.
  • Since the label is a property of the filesystem, it is not suitable for addressing a single RAID device persistently.

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 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 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).

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

by-id と 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), 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-partlabel

ノート: この方法は GUID Partition Table (GPT) のディスクでだけ使えます。

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

See also Wikipedia:GUID Partition Table#Partition entries.

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
ノート:
  • GPT partition labels have also to be different to avoid conflicts. To change your partition label, you can use gdisk or the ncurse-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

ノート: この方法は GUID Partition Table (GPT) のディスクでだけ使えます。

Like GPT partition labels, GPT partition UUID are defined in the partition entry on GPT disks.

Wikipedia:ja:GUIDパーティションテーブル#パーティションエントリ (LBA 2〜33) も見て下さい。

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

Udev によるデバイス名の固定

Udev#固定デバイス名の設定 を見て下さい。

永続的な命名の使用

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

fstab

次の記事を参照してください: fstab#UUID。

ブートマネージャ

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 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

or: 

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 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 GRUB: 

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