The Linux kernel has a notion of device mapping: a block device, such
as a hard disk partition, can be mapped into another device, usually
/dev/mapper/, with additional processing over the data that flows
through it28. A typical example is encryption
device mapping: all writes to the mapped device are encrypted, and all reads
are deciphered, transparently. Guix extends this notion by considering any
device or set of devices that are transformed in some way to create a
new device; for instance, RAID devices are obtained by assembling
several other devices, such as hard disks or partitions, into a new one that
behaves as one partition.
Mapped devices are declared using the
mapped-device form, defined as
follows; for examples, see below.
Objects of this type represent device mappings that will be made when the system boots up.
This is either a string specifying the name of the block device to be
mapped, such as
"/dev/sda3", or a list of such strings when several
devices need to be assembled for creating a new one. In case of LVM this is
a string specifying name of the volume group to be mapped.
This string specifies the name of the resulting mapped device. For kernel
mappers such as encrypted devices of type
"my-partition" leads to the creation of the
"/dev/mapper/my-partition" device. For RAID devices of type
raid-device-mapping, the full device name such as
needs to be given. LVM logical volumes of type
need to be specified as
This list of strings specifies names of the resulting mapped devices in case there are several. The format is identical to target.
This must be a
mapped-device-kind object, which specifies how
source is mapped to target.
This defines LUKS block device encryption using the
command from the package with the same name. It relies on the
dm-crypt Linux kernel module.
This defines a RAID device, which is assembled using the
command from the package with the same name. It requires a Linux kernel
module for the appropriate RAID level to be loaded, such as
for RAID-4, RAID-5 or RAID-6, or
raid10 for RAID-10.
This defines one or more logical volumes for the Linux
Logical Volume Manager (LVM). The
volume group is activated by the
vgchange command from the
The following 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
(mapped-device (source "/dev/sda3") (target "home") (type luks-device-mapping))
Alternatively, to become independent of device numbering, one may obtain the LUKS UUID (unique identifier) of the source device by a command like:
cryptsetup luksUUID /dev/sda3
and use it as follows:
(mapped-device (source (uuid "cb67fc72-0d54-4c88-9d4b-b225f30b0f44")) (target "home") (type luks-device-mapping))
It is also desirable to encrypt swap space, since swap space may contain sensitive data. One way to accomplish that is to use a swap file in a file system on a device mapped via LUKS encryption. In this way, the swap file is encrypted because the entire device is encrypted. See Swap Space, or See Disk Partitioning, for an example.
A RAID device formed of the partitions /dev/sda1 and /dev/sdb1 may be declared as follows:
(mapped-device (source (list "/dev/sda1" "/dev/sdb1")) (target "/dev/md0") (type raid-device-mapping))
The /dev/md0 device can then be used as the
device of a
file-system declaration (see 文件系统). Note that the RAID
level need not be given; it is chosen during the initial creation and
formatting of the RAID device and is determined automatically later.
LVM logical volumes “alpha” and “beta” from volume group “vg0” can be declared as follows:
(mapped-device (source "vg0") (targets (list "vg0-alpha" "vg0-beta")) (type lvm-device-mapping))
Devices /dev/mapper/vg0-alpha and /dev/mapper/vg0-beta can
then be used as the
device of a
Note that the GNU Hurd makes no difference between the concept of a “mapped device” and that of a file system: both boil down to translating input/output operations made on a file to operations on its backing store. Thus, the Hurd implements mapped devices, like file systems, using the generic translator mechanism (see Translators in The GNU Hurd Reference Manual).