12.4 Dispositivos traducidos

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 in /dev/mapper/, with additional processing over the data that flows through it³⁴. 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.

Los dispositivos traducidos se declaran mediante el uso de la forma mapped-device, definida a continuación; ejemplos más adelante.

Tipo de datos: mapped-device

Objetos de este tipo representan traducciones de dispositivo que se llevarán a cabo cuando el sistema arranque.

source

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.

target

This string specifies the name of the resulting mapped device. For kernel mappers such as encrypted devices of type luks-device-mapping, specifying "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 "/dev/md0" needs to be given. LVM logical volumes of type lvm-device-mapping need to be specified as "VGNAME-LVNAME".

targets

This list of strings specifies names of the resulting mapped devices in case there are several. The format is identical to target.

type

Debe ser un objeto mapped-device-kind, que especifica cómo source se traduce a target.

Variable Scheme: luks-device-mapping

Define el cifrado de bloques LUKS mediante el uso de la orden cryptsetup del paquete del mismo nombre. Depende del módulo dm-crypt del núcleo Linux.

Variable Scheme: raid-device-mapping

Define un dispositivo RAID, el cual se ensambla mediante el uso de la orden mdadm del paquete del mismo nombre. Requiere la carga del módulo del núcleo Linux para el nivel RAID apropiado, como raid456 para RAID-4, RAID-5 o RAID-6, o raid10 para RAID-10.

Scheme Variable: lvm-device-mapping

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 lvm2 package.

El siguiente ejemplo especifica una traducción de /dev/sda3 a /dev/mapper/home mediante el uso de LUKS—la configuración de claves unificada de Linux, un mecanismo estándar para cifrado de disco. El dispositivo /dev/mapper/home puede usarse entonces como el campo device de una declaración file-system (see Sistemas de archivos).

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

De manera alternativa, para independizarse de la numeración de dispositivos, puede obtenerse el UUID LUKS (identificador único) del dispositivo fuente con una orden así:

cryptsetup luksUUID /dev/sda3

y usarlo como sigue:

(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.

Un dispositivo RAID formado por las particiones /dev/sda1 y /dev/sdb1 puede declararse como se muestra a continuación:

(mapped-device
  (source (list "/dev/sda1" "/dev/sdb1"))
  (target "/dev/md0")
  (type raid-device-mapping))

El dispositivo /dev/md0 puede usarse entonces como el campo device de una declaración file-system (see Sistemas de archivos). Fíjese que no necesita proporcionar el nivel RAID; se selecciona durante la creación inicial y formato del dispositivo RAID y después se determina automáticamente.

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 file-system declaration (see Sistemas de archivos).