Chapter 6. Overview of persistent naming attributes

• The parallelization of the system boot process detects storage devices in a different order with each system boot.
• A disk fails to power up or respond to the SCSI controller. This results in it not being detected by the normal device probe. The disk is not accessible to the system and subsequent devices will have their major and minor number range, including the associated sd names shifted down. For example, if a disk normally referred to as sdb is not detected, a disk that is normally referred to as sdc would instead appear as sdb.
• A SCSI controller (host bus adapter, or HBA) fails to initialize, causing all disks connected to that HBA to not be detected. Any disks connected to subsequently probed HBAs are assigned different major and minor number ranges, and different associated sd names.
• The order of driver initialization changes if different types of HBAs are present in the system. This causes the disks connected to those HBAs to be detected in a different order. This might also occur if HBAs are moved to different PCI slots on the system.
• Disks connected to the system with Fibre Channel, iSCSI, or FCoE adapters might be inaccessible at the time the storage devices are probed, due to a storage array or intervening switch being powered off, for example. This might occur when a system reboots after a power failure, if the storage array takes longer to come online than the system take to boot. Although some Fibre Channel drivers support a mechanism to specify a persistent SCSI target ID to WWPN mapping, this does not cause the major and minor number ranges, and the associated sd names to be reserved; it only provides consistent SCSI target ID numbers.

• Unique identifier (UUID)
• Label

• World Wide Identifier (WWID)
• Partition UUID
• Serial number

• Some file systems, such as logical volumes, span multiple devices. Red Hat recommends accessing these file systems using file system identifiers rather than device identifiers.

• Their content
• A unique identifier
• Their serial number.

• Host:Channel:Target:LUN
• /dev/sd name
• major:minor number

• It is possible that the device might not be accessible at the time the query is performed because the udev mechanism might rely on the ability to query the storage device when the udev rules are processed for a udev event. This is more likely to occur with Fibre Channel, iSCSI or FCoE storage devices when the device is not located in the server chassis.
• The kernel might send udev events at any time, causing the rules to be processed and possibly causing the /dev/disk/by-*/ links to be removed if the device is not accessible.
• There might be a delay between when the udev event is generated and when it is processed, such as when a large number of devices are detected and the user-space udevd service takes some amount of time to process the rules for each one. This might cause a delay between when the kernel detects the device and when the /dev/disk/by-*/ names are available.
• External programs such as blkid invoked by the rules might open the device for a brief period of time, making the device inaccessible for other uses.
• The device names managed by the udev mechanism in /dev/disk/ may change between major releases, requiring you to update the links.

Procedure

• To list the UUID and Label attributes, use the lsblk utility:

  $ lsblk --fs storage-device

  For example:

  Example 6.4. Viewing the UUID and Label of a file system

  $ lsblk --fs /dev/sda1
  
  NAME FSTYPE LABEL UUID                                 MOUNTPOINT
  sda1 xfs    Boot  afa5d5e3-9050-48c3-acc1-bb30095f3dc4 /boot

• To list the PARTUUID attribute, use the lsblk utility with the --output +PARTUUID option:

  $ lsblk --output +PARTUUID

  For example:

  Example 6.5. Viewing the PARTUUID attribute of a partition

  $ lsblk --output +PARTUUID /dev/sda1
  
  NAME MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT PARTUUID
  sda1   8:1    0  512M  0 part /boot      4cd1448a-01

• To list the WWID attribute, examine the targets of symbolic links in the /dev/disk/by-id/ directory. For example:

  Example 6.6. Viewing the WWID of all storage devices on the system

  $ file /dev/disk/by-id/*
  
  /dev/disk/by-id/ata-QEMU_HARDDISK_QM00001
  symbolic link to ../../sda
  /dev/disk/by-id/ata-QEMU_HARDDISK_QM00001-part1
  symbolic link to ../../sda1
  /dev/disk/by-id/ata-QEMU_HARDDISK_QM00001-part2
  symbolic link to ../../sda2
  /dev/disk/by-id/dm-name-rhel_rhel8-root
  symbolic link to ../../dm-0
  /dev/disk/by-id/dm-name-rhel_rhel8-swap
  symbolic link to ../../dm-1
  /dev/disk/by-id/dm-uuid-LVM-QIWtEHtXGobe5bewlIUDivKOz5ofkgFhP0RMFsNyySVihqEl2cWWbR7MjXJolD6g
  symbolic link to ../../dm-1
  /dev/disk/by-id/dm-uuid-LVM-QIWtEHtXGobe5bewlIUDivKOz5ofkgFhXqH2M45hD2H9nAf2qfWSrlRLhzfMyOKd
  symbolic link to ../../dm-0
  /dev/disk/by-id/lvm-pv-uuid-atlr2Y-vuMo-ueoH-CpMG-4JuH-AhEF-wu4QQm
  symbolic link to ../../sda2

• Replace new-uuid with the UUID you want to set; for example, 1cdfbc07-1c90-4984-b5ec-f61943f5ea50. You can generate a UUID using the uuidgen command.
• Replace new-label with a label; for example, backup_data.

Prerequisites

• If you are modifying the attributes of an XFS file system, unmount it first.

Procedure

• To change the UUID or Label attributes of an XFS file system, use the xfs_admin utility:

  # xfs_admin -U new-uuid -L new-label storage-device
  # udevadm settle

• To change the UUID or Label attributes of an ext4, ext3, or ext2 file system, use the tune2fs utility:

  # tune2fs -U new-uuid -L new-label storage-device
  # udevadm settle

• To change the UUID or Label attributes of a swap volume, use the swaplabel utility:

  # swaplabel --uuid new-uuid --label new-label swap-device
  # udevadm settle