Device file

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Reason: Explain loop devices. (Discuss in Talk:Device file)

From Wikipedia:

In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file.

On Linux they are in the /dev directory, according to the Filesystem Hierarchy Standard.

On Arch Linux the device nodes are managed by udev.

Block devices

A block device is a special file that provides buffered access to a hardware device. For a detailed description and comparison of virtual file system devices, see Wikipedia:Device file#Block devices.

Block device names

The beginning of the device name specifies the kernel's used driver subsystem to operate the block device.

Warning: Kernel name descriptors for block devices are not persistent and can change each boot, they should not be used in configuration files.

SCSI

Storage devices, like hard disks, SSDs and flash drives, that support the SCSI command (SCSI, SAS, UASP), ATA (PATA, SATA) or USB mass storage connection are handled by the kernel's SCSI driver subsystem. They all share the same naming scheme.

The name of these devices starts with sd. It is then followed by a lower-case letter starting from a for the first discovered device (sda), b for the second discovered device (sdb), and so on.

Examples:

  • /dev/sda - device a, the first discovered device.
  • /dev/sde - device e, the fifth discovered device.

NVMe

The name of storage devices, like SSDs, that are attached via NVM Express (NVMe) starts with nvme. It is then followed by a number starting from 0 for the device controller, nvme0 for the first discovered NVMe controller, nvme1 for the second, and so on. Next is the letter "n" and a number starting from 1 expressing the device on a controller, i.e. nvme0n1 for first discovered device on first discovered controller, nvme0n2 for second discovered device on first discovered controller, and so on.

Examples:

  • /dev/nvme0n1 - device 1 on controller 0, the first discovered device on the first discovered controller.
  • /dev/nvme2n5 - device 5 on controller 2, the fifth discovered device on the third discovered controller.

MMC

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Reason: Mention /dev/mmcblkXboot{0,1}[1] and /dev/mmcblkXrpmb[2]. (Discuss in Talk:Partitioning#mmcblk0p{1,2,3,4}, mmcblk0boot{0,1}, mmcblk0rpmb)

SD cards, MMC cards and eMMC storage devices are handled by the kernel's mmc driver and name of those devices start with mmcblk. It is then followed by a number starting from 0 for the device, i.e. mmcblk0 for first discovered device, mmcblk1 for second discovered device and so on.

Examples:

  • /dev/mmcblk0 - device 0, the first discovered device.
  • /dev/mmcblk4 - device 4, the fifth discovered device.

SCSI optical disc drive

The name of optical disc drives (ODDs), that are attached using one of the interfaces supported by the SCSI driver subsystem, start with sr. The name is then followed by a number starting from 0 for the device, ie. sr0 for the first discovered device, sr1 for the second discovered device, and so on.

Udev also provides /dev/cdrom that is a symbolic link to /dev/sr0. The name will always be cdrom regardless of the drive's supported disc types or the inserted media.

Examples:

  • /dev/sr0 - optical disc drive 0, the first discovered optical disc drive.
  • /dev/sr4 - optical disc drive 4, the fifth discovered optical disc drive.
  • /dev/cdrom - a symbolic link to /dev/sr0.

virtio-blk

The name of drives attached to a virtio block device (virtio-blk) interface start with vd. It is then followed by a lower-case letter starting from a for the first discovered device (vda), b for the second discovered device (vdb), and so on.

Note: Do no confuse virtio-blk with virtio-scsi which emulates a SCSI controller and thus follows the SCSI naming convention.

Examples:

  • /dev/vda - device a, the first discovered device.
  • /dev/vde - device e, the fifth discovered device.

Partition

Partition device names are a combination of the drive's device name and the partition number assigned to them in the partition table, i.e. /dev/drivepartition. For drives whose device name ends with a number, the drive name and partition number is separated with the letter "p", i.e. /dev/driveppartition.

Examples:

  • /dev/sda1 - partition 1 on /dev/sda.
  • /dev/nvme2n5p3 - partition 3 on /dev/nvme2n5.
  • /dev/mmcblk3p4 - partition 4 on /dev/mmcblk3.
  • /dev/vda1 - partition 1 on /dev/vda.
  • /dev/loop0p2 - partition 2 on /dev/loop0.

Utilities

lsblk

The util-linux package provides the lsblk(8) utility which lists block devices, for example: 

$ lsblk -f
NAME   FSTYPE   LABEL       UUID                                 MOUNTPOINT
sda
├─sda1 vfat                 C4DA-2C4D                            /boot
├─sda2 swap                 5b1564b2-2e2c-452c-bcfa-d1f572ae99f2 [SWAP]
└─sda3 ext4                 56adc99b-a61e-46af-aab7-a6d07e504652 /

In the example above, only one device is available (sda), and that device has three partitions (sda1 to sda3), each with a different file system.

wipefs

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Reason: Why would you want to make signatures invisible for libblkid? (Discuss in Talk:Device file)

wipefs can list or erase file system, RAID or partition-table signatures (magic strings) from the specified device to make the signatures invisible for libblkid(3). It does not erase the file systems themselves nor any other data from the device.

See wipefs(8) for more information.

For example, to erase all signatures from the device /dev/sdb and create a signature backup ~/wipefs-sdb-offset.bak file for each signature: 

# wipefs --all --backup /dev/sdb

Pseudo-devices

Device nodes that do not have a physical device.

See also