Chapter 6. Overview of persistent naming attributes
Chapter 6. Overview of persistent naming attributes As a system administrator, you need to refer to storage volumes using persistent naming attributes to build storage setups that are reliable over multiple system boots. 6.1. Disadvantages of non-persistent naming attributes Red Hat Enterprise Linux provides a number of ways to identify storage devices. It is important to use the correct option to identify each device when used in order to avoid inadvertently accessing the wrong device, particularly when installing to or reformatting drives. Traditionally, non-persistent names in the form of /dev/sd (major number) (minor number) are used on Linux to refer to storage devices. The major and minor number range and associated sd names are allocated for each device when it is detected. This means that the association between the major and minor number range and associated sd names can change if the order of device detection changes. Such a change in the ordering might occur in the following situations: 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. These reasons make it undesirable to use the major and minor number range or the associated sd names when referring to devices, such as in the /etc/fstab file. There is the possibility that the wrong device will be mounted and data corruption might result. Occasionally, however, it is still necessary to refer to the sd names even when another mechanism is used, such as when errors are reported by a device. This is because the Linux kernel uses sd names (and also SCSI host/channel/target/LUN tuples) in kernel messages regarding the device. 6.2. File system and device identifiers File system identifiers are tied to the file system itself, while device identifiers are linked to the physical block device. Understanding the difference is important for proper storage management. File system identifiers File system identifiers are tied to a particular file system created on a block device. The identifier is also stored as part of the file system. If you copy the file system to a different device, it still carries the same file system identifier. However, if you rewrite the device, such as by formatting it with the mkfs utility, the device loses the attribute. File system identifiers include: Unique identifier (UUID) Label Device identifiers Device identifiers are tied to a block device: for example, a disk or a partition. If you rewrite the device, such as by formatting it with the mkfs utility, the device keeps the attribute, because it is not stored in the file system. Device identifiers include: World Wide Identifier (WWID) Partition UUID Serial number Recommendations 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. 6.3. Device names managed by the udev mechanism in /dev/disk/ The udev mechanism is used for all types of devices in Linux, and is not limited only for storage devices. It provides different kinds of persistent naming attributes in the /dev/disk/ directory. In the case of storage devices, Red Hat Enterprise Linux contains udev rules that create symbolic links in the /dev/disk/ directory. This enables you to refer to storage devices by: Their content A unique identifier Their serial number. Although udev naming attributes are persistent, in that they do not change on their own across system reboots, some are also configurable. 6.3.1. File system identifiers The UUID attribute in /dev/disk/by-uuid/ Entries in this directory provide a symbolic name that refers to the storage device by a unique identifier (UUID) in the content (that is, the data) stored on the device. For example: You can use the UUID to refer to the device in the /etc/fstab file using the following syntax: You can configure the UUID attribute when creating a file system, and you can also change it later on. The Label attribute in /dev/disk/by-label/ Entries in this directory provide a symbolic name that refers to the storage device by a label in the content (that is, the data) stored on the device. For example: You can use the label to refer to the device in the /etc/fstab file using the following syntax: You can configure the Label attribute when creating a file system, and you can also change it later on. 6.3.2. Device identifiers The WWID attribute in /dev/disk/by-id/ The World Wide Identifier (WWID) is a persistent, system-independent identifier that the SCSI Standard requires from all SCSI devices. The WWID identifier is guaranteed to be unique for every storage device, and independent of the path that is used to access the device. The identifier is a property of the device but is not stored in the content (that is, the data) on the devices. This identifier can be obtained by issuing a SCSI Inquiry to retrieve the Device Identification Vital Product Data (page 0x83 ) or Unit Serial Number (page 0x80 ). Red Hat Enterprise Linux automatically maintains the proper mapping from the WWID-based device name to a current /dev/sd name on that system. Applications can use the /dev/disk/by-id/ name to reference the data on the disk, even if the path to the device changes, and even when accessing the device from different systems. Note If your are using an NVMe device, you might run into a disk by-id naming change for some vendors, if the serial number of your device has leading whitespace. Example 6.1. WWID mappings WWID symlink Non-persistent device Note /dev/disk/by-id/scsi-3600508b400105e210000900000490000 /dev/sda A device with a page 0x83 identifier /dev/disk/by-id/scsi-SSEAGATE_ST373453LW_3HW1RHM6 /dev/sdb A device with a page 0x80 identifier /dev/disk/by-id/ata-SAMSUNG_MZNLN256HMHQ-000L7_S2WDNX0J336519-part3 /dev/sdc3 A disk partition In addition to these persistent names provided by the system, you can also use udev rules to implement persistent names of your own, mapped to the WWID of the storage. The Partition UUID attribute in /dev/disk/by-partuuid The Partition UUID (PARTUUID) attribute identifies partitions as defined by GPT partition table. Example 6.2. Partition UUID mappings PARTUUID symlink Non-persistent device /dev/disk/by-partuuid/4cd1448a-01 /dev/sda1 /dev/disk/by-partuuid/4cd1448a-02 /dev/sda2 /dev/disk/by-partuuid/4cd1448a-03 /dev/sda3 The Path attribute in /dev/disk/by-path/ This attribute provides a symbolic name that refers to the storage device by the hardware path used to access the device. The Path attribute fails if any part of the hardware path (for example, the PCI ID, target port, or LUN number) changes. The Path attribute is therefore unreliable. However, the Path attribute may be useful in one of the following scenarios: You need to identify a disk that you are planning to replace later. You plan to install a storage service on a disk in a specific location. 6.4. The World Wide Identifier with DM Multipath You can configure Device Mapper (DM) Multipath to map between the World Wide Identifier (WWID) and non-persistent device names. If there are multiple paths from a system to a device, DM Multipath uses the WWID to detect this. DM Multipath then presents a single "pseudo-device" in the /dev/mapper/wwid directory, such as /dev/mapper/3600508b400105df70000e00000ac0000 . The command multipath -l shows the mapping to the non-persistent identifiers: Host : Channel : Target : LUN /dev/sd name major : minor number Example 6.3. WWID mappings in a multipath configuration An example output of the multipath -l command: DM Multipath automatically maintains the proper mapping of each WWID-based device name to its corresponding /dev/sd name on the system. These names are persistent across path changes, and they are consistent when accessing the device from different systems. When the user_friendly_names feature of DM Multipath is used, the WWID is mapped to a name of the form /dev/mapper/mpath N . By default, this mapping is maintained in the file /etc/multipath/bindings . These mpath N names are persistent as long as that file is maintained. Important If you use user_friendly_names , then additional steps are required to obtain consistent names in a cluster. 6.5. Limitations of the udev device naming convention The following are some limitations of the udev naming convention: 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. 6.6. Listing persistent naming attributes You can find out the persistent naming attributes of non-persistent storage devices. Procedure To list the UUID and Label attributes, use the lsblk utility: For example: Example 6.4. Viewing the UUID and Label of a file system To list the PARTUUID attribute, use the lsblk utility with the --output +PARTUUID option: For example: Example 6.5. Viewing the PARTUUID attribute of a partition 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 6.7. Modifying persistent naming attributes You can change the UUID or Label persistent naming attribute of a file system. Note Changing udev attributes happens in the background and might take a long time. The udevadm settle command waits until the change is fully registered, which ensures that your command will be able to use the new attribute correctly. In the following commands: 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: To change the UUID or Label attributes of an ext4 , ext3 , or ext2 file system, use the tune2fs utility: To change the UUID or Label attributes of a swap volume, use the swaplabel utility:
/dev/disk/by-uuid/ 3e6be9de-8139-11d1-9106-a43f08d823a6 UUID= 3e6be9de-8139-11d1-9106-a43f08d823a6 /dev/disk/by-label/ Boot LABEL= Boot 3600508b400105df70000e00000ac0000 dm-2 vendor,product [size=20G][features=1 queue_if_no_path][hwhandler=0][rw] \_ round-robin 0 [prio=0][active] \_ 5:0:1:1 sdc 8:32 [active][undef] \_ 6:0:1:1 sdg 8:96 [active][undef] \_ round-robin 0 [prio=0][enabled] \_ 5:0:0:1 sdb 8:16 [active][undef] \_ 6:0:0:1 sdf 8:80 [active][undef] lsblk --fs storage-device lsblk --fs /dev/sda1 NAME FSTYPE LABEL UUID MOUNTPOINT sda1 xfs Boot afa5d5e3-9050-48c3-acc1-bb30095f3dc4 /boot lsblk --output +PARTUUID lsblk --output +PARTUUID /dev/sda1 NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT PARTUUID sda1 8:1 0 512M 0 part /boot 4cd1448a-01 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 xfs_admin -U new-uuid -L new-label storage-device udevadm settle tune2fs -U new-uuid -L new-label storage-device udevadm settle swaplabel --uuid new-uuid --label new-label swap-device udevadm settle