Tutorial: building disk images

In this tutorial we will show you what properties you need to use to build bootable disk images.

Tutorial conventions

In these tutorials we assume that you have a workstation called laptop.example.com where you run the root Lisp. We also assume that Consfigurator knows about your laptop, and that it has a host deployment specified, so that you can use HOSTDEPLOY-THESE to deploy properties to the laptop as root. For example,:

(defhost laptop.example.com
    (:deploy ((:sudo :as "spwhitton@laptop.example.com") :sbcl))
  "Sean's laptop."
  (os:debian-stable "bullseye" :amd64))

We suppose that you’ve already set up sources of prerequisite data to provide sudo passwords and the like. See the introduction if you haven’t set this up yet.

Consfiguration

Here is a minimal definition of the host for which we can build a disk image::

(defhost test.example.com ()
  (os:debian-stable "bullseye" :amd64)
  (disk:has-volumes
   (physical-disk
    :device-file #P"/dev/sda"
    :boots-with '(grub:grub :target "x86_64-efi" :force-extra-removable t)
    (partitioned-volume
     ((partition
       :partition-typecode #xEF00
       (fat32-filesystem :volume-size 512 :mount-point #P"/boot/efi/"))
      (partition
       (ext4-filesystem :extra-space 400 :mount-point #P"/"))))))
  (installer:bootloader-binaries-installed)
  (apt:installed "linux-image-amd64")
  (user:has-enabled-password "root"))
  • The DISK:HAS-VOLUMES property is like the OS:DEBIAN-STABLE property in that both simply set hostattrs on the host – they establish metadata to which other properties may refer. In this case, we specify that the machine has a single physical disk with two partitions, and that it boots with GRUB. We also request that Consfigurator pass the --force-extra-removable flag to grub-install(8), because that makes it a bit easier to test our image, given how UEFI works.

    We’ve requested 400M of free space on the root partition beyond whatever the base system install takes up. For the EFI system partition we specify an absolute size.

  • The INSTALLER:BOOTLOADER-BINARIES-INSTALLED property reads the metadata established by DISK:HAS-VOLUMES and ensures that binaries like grub-install(8) are available. In this case you could replace it with just (apt:installed "grub-efi-amd64"), but using this property avoids repeating yourself.

  • Finally, building a bootable image requires installing a kernel.

Building the image

What we’ve established so far is a definition of a host. But it does not yet make any sense to say (deploy :foo test.example.com ...) because the host does not yet exist anywhere for us to connect to it. What we can now use is the DISK:RAW-IMAGE-BUILT-FOR property, which we can apply to a host which does already exist to build an image for our host which does not yet exist::

CONSFIG> (hostdeploy-these laptop.example.com
           (disk:raw-image-built-for
            nil test.example.com "/home/spwhitton/tmp/test.img"))

This property does the following on laptop.example.com:

  1. Build a chroot with the root filesystem of test.example.com, and apply all its properties, such as installing the kernel and building the initramfs.

  2. Transform the metadata set by DISK:HAS-VOLUMES such that the instance of PHYSICAL-DISK is replaced with an instance of RAW-DISK-IMAGE, and then make, partition and mount the image file, using tools like kpartx(8).

  3. Rsync the contents of the chroot into the mounted partitions.

  4. Update /etc/fstab so that it contains the UUIDs of the partitions.

  5. Install the bootloader(s), again as specified by DISK:HAS-VOLUMES.

Here we’ve described this procedurally, but the semantics of DISK:RAW-IMAGE-BUILT-FOR are declarative, like all properties. You can add the property to the DEFHOST for your laptop and Consfigurator will just do whatever is needed to keep the chroot and the disk image up-to-date (though see the docstring for that property for some limitations).

All of this is modular: take a look in src/property/disk.lisp to see how new volume types can be defined, and in src/property/grub.lisp and src/property/u-boot.lisp to see how Consfigurator can be taught to install different bootloaders.

Testing the image

Here’s a quick way to test what we’ve built::

% sudo chown $USER tmp/test.img
% qemu-system-x86_64 -m 2G -drive file=tmp/test.img,format=raw \
    -drive "if=pflash,format=raw,readonly=on,file=/usr/share/OVMF/OVMF_CODE.fd"

It should boot up and you can login as root, password “changeme”.

Uses for the disk image

You might upload this image to a cloud provider and boot up a minimal instance. Supposing we also added at least an sshd and our public key, we could then continue to add properties to the DEFHOST for test.example.com and then apply them with SSH::

CONSFIG> (deploy ((:ssh :user "root") :sbcl) test.example.com)

Another possibility is to dd the image out to a USB flash drive and then boot a physical machine from it.

It should be straightforward to adapt the existing code to have Consfigurator install a bootable system to a physical volume rather than to a disk image, but the high level properties for this haven’t been written yet.