SCSI device nodes

SCSI device names are assigned in the order in which the devices are detected. In a typical SAN environment, this can mean a seemingly arbitrary mapping of names to actual devices that can change between boots. Therefore, using standard device nodes of the form /dev/<device_name> where <device_name> is the device name that the SCSI stack assigns to a device, can be a challenge.

Ubuntu Server 20.04.1 LTS provides udev to create device nodes for you. Use the device nodes to identify the corresponding actual device.

Device nodes that are based on device names

udev creates device nodes that match the device names that are used by the kernel. These standard device nodes have the form /dev/<name>.

The examples in this section use standard device nodes as assigned by the SCSI stack. These nodes have the form /dev/sd<x> for entire disks and /dev/sd<x><n> for partitions. In these node names <x> represents one or more letters and <n> is an integer. For more information about the SCSI device naming scheme, see the /linux-doc/admin-guide/devices.txt.gz file in the linux-doc package.

To help you identify a particular device, udev creates additional device nodes that are based on the device's bus ID, the device label, and information about the file system on the device. The file system information can be a universally unique identifier (UUID) and, if available, the file system label.

Device nodes that are based on bus IDs

udev creates device nodes of the form

/dev/disk/by-path/ccw-<device_bus_id>-fc-<wwpn>-lun-<lun>

for whole SCSI device and

/dev/disk/by-path/ccw-<device_bus_id>-fc-<wwpn>-lun-<lun>-part<n>

for the <n>th partition, where <wwpn> is the worldwide port number of the target port and <lun> is the logical unit number that represents the target SCSI device.

Device nodes that are based on file system information

udev creates device nodes of the form

/dev/disk/by-uuid/<uuid>

where <uuid> is a unique file-system identifier (UUID) for the file system in a partition.

If a file system label is assigned, udev also creates a node of the form:

/dev/disk/by-label/<label>

There are no device nodes for the whole SCSI device that are based on file system information.

Additional device nodes

/dev/disk/by-id contains additional device nodes for the SCSI device and partitions that are all based on a unique SCSI identifier that is generated by querying the device.