7.2.4 Mapped Devices

The Linux kernel has a notion of device mapping: a block device, such as a hard disk partition, can be mapped into another device, with additional processing over the data that flows through it¹⁷. A typical example is encryption device mapping: all writes to the mapped device are encrypted, and all reads are deciphered, transparently.

Mapped devices are declared using the mapped-device form:

(mapped-device
  (source "/dev/sda3")
  (target "home")
  (type luks-device-mapping))

This example specifies a mapping from /dev/sda3 to /dev/mapper/home using LUKS—the Linux Unified Key Setup, a standard mechanism for disk encryption. The /dev/mapper/home device can then be used as the device of a file-system declaration (see File Systems). The mapped-device form is detailed below.

Data Type: mapped-device

Objects of this type represent device mappings that will be made when the system boots up.

source

This string specifies the name of the block device to be mapped, such as "/dev/sda3".

target

This string specifies the name of the mapping to be established. For example, specifying "my-partition" will lead to the creation of the "/dev/mapper/my-partition" device.

type

This must be a mapped-device-kind object, which specifies how source is mapped to target.

Scheme Variable: luks-device-mapping

This defines LUKS block device encryption using the cryptsetup command, from the same-named package. This relies on the dm-crypt Linux kernel module.