16.2 Instantiate an Image

Let’s say you would like to create an MBR image with three distinct partitions:

• The ESP (EFI System Partition), a partition of 40 MiB at offset 1024 KiB with a vfat file system.
• an ext4 partition of 50 MiB data file, and labeled “data”.
• an ext4 bootable partition containing the %simple-os operating-system.

You would then write the following image definition in a my-image.scm file for instance.

(use-modules (gnu)
             (gnu image)
             (gnu tests)
             (gnu system image)
             (guix gexp))

(define MiB (expt 2 20))

(image
 (format 'disk-image)
 (operating-system %simple-os)
 (partitions
  (list
   (partition
    (size (* 40 MiB))
    (offset (* 1024 1024))
    (label "GNU-ESP")
    (file-system "vfat")
    (flags '(esp))
    (initializer (gexp initialize-efi-partition)))
   (partition
    (size (* 50 MiB))
    (label "DATA")
    (file-system "ext4")
    (initializer #~(lambda* (root . rest)
                     (mkdir root)
                     (call-with-output-file
                         (string-append root "/data")
                       (lambda (port)
                         (format port "my-data"))))))
   (partition
    (size 'guess)
    (label root-label)
    (file-system "ext4")
    (flags '(boot))
    (initializer (gexp initialize-root-partition))))))

Note that the first and third partitions use generic initializers procedures, initialize-efi-partition and initialize-root-partition respectively. The initialize-efi-partition installs a GRUB EFI loader that is loading the GRUB bootloader located in the root partition. The initialize-root-partition instantiates a complete system as defined by the %simple-os operating-system.

You can now run:

guix system image my-image.scm

to instantiate the image definition. That produces a disk image which has the expected structure:

$ parted $(guix system image my-image.scm) print
…
Model:  (file)
Disk /gnu/store/yhylv1bp5b2ypb97pd3bbhz6jk5nbhxw-disk-image: 1714MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  43.0MB  41.9MB  primary  fat16        esp
 2      43.0MB  95.4MB  52.4MB  primary  ext4
 3      95.4MB  1714MB  1619MB  primary  ext4         boot

The size of the boot partition has been inferred to 1619MB so that it is large enough to host the %simple-os operating-system.

You can also use existing image record definitions and inherit from them to simplify the image definition. The (gnu system image) module provides the following image definition variables.

Variable: efi-disk-image

MBR 磁盘镜像由两个分区组成：一个 64 位的 ESP 分区和一个 ROOT 引导分区。这种镜像可以在大多数 x86_64 和 i686 机器上使用，支持 BIOS 或 UEFI 引导。

Variable: efi32-disk-image

与 efi-disk-image 相同，但是有一个 32 位的 EFI 分区。

Variable: iso9660-image

ISO-9660镜像由一个可引导分区组成。这种镜像可以在大多数 x86_64 和 i686 机器上使用。

Variable: docker-image

Docker 镜像可以用来孵化 Docker 容器。

使用 efi-disk-image 我们就可以像这样简化先前的 image 声明：

(use-modules (gnu)
             (gnu image)
             (gnu tests)
             (gnu system image)
             (guix gexp)
             (ice-9 match))

(define MiB (expt 2 20))

(define data
  (partition
   (size (* 50 MiB))
   (label "DATA")
   (file-system "ext4")
   (initializer #~(lambda* (root . rest)
                    (mkdir root)
                    (call-with-output-file
                        (string-append root "/data")
                      (lambda (port)
                        (format port "my-data")))))))

(image
 (inherit efi-disk-image)
 (operating-system %simple-os)
 (partitions
  (match (image-partitions efi-disk-image)
    ((esp root)
     (list esp data root)))))

这将会有一模一样的 image 实例化，不过 image 声明更加简单。