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<nixpkgs/nixos/lib/make-disk-image.nix>

<nixpkgs/nixos/lib/make-disk-image.nix> is a function to create disk images in multiple formats: raw, QCOW2 (QEMU), QCOW2-Compressed (compressed version), VDI (VirtualBox), VPC (VirtualPC).

This function can create images in two ways:

  • using cptofs without any virtual machine to create a Nix store disk image,
  • using a virtual machine to create a full NixOS installation.

When testing early-boot or lifecycle parts of NixOS such as a bootloader or multiple generations, it is necessary to opt for a full NixOS system installation. Whereas for many web servers, applications, it is possible to work with a Nix store only disk image and is faster to build.

NixOS tests also use this function when preparing the VM. The cptofs method is used when virtualisation.useBootLoader is false (the default). Otherwise the second method is used.

Features

For reference, read the function signature source code for documentation on arguments: https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-disk-image.nix. Features are separated in various sections depending on if you opt for a Nix-store only image or a full NixOS image.

Common

  • arbitrary NixOS configuration
  • automatic or bound disk size: diskSize parameter, additionalSpace can be set when diskSize is auto to add a constant of disk space
  • multiple partition table layouts: EFI, legacy, legacy + GPT, hybrid, none through partitionTableType parameter
  • OVMF or EFI firmwares and variables templates can be customized
  • root filesystem fsType can be customized to whatever mkfs.${fsType} exist during operations
  • root filesystem label can be customized, defaults to nix-store if it's a Nix store image, otherwise nixpkgs/nixos
  • arbitrary code can be executed after disk image was produced with postVM
  • the current nixpkgs can be realized as a channel in the disk image, which will change the hash of the image when the sources are updated
  • additional store paths can be provided through additionalPaths

Full NixOS image

  • arbitrary contents with permissions can be placed in the target filesystem using contents
  • a /etc/nixpkgs/nixos/configuration.nix can be provided through configFile
  • bootloaders are supported
  • EFI variables can be mutated during image production and the result is exposed in $out
  • boot partition size when partition table is efi or hybrid

On bit-to-bit reproducibility

Images are NOT deterministic, please do not hesitate to try to fix this, source of determinisms are (not exhaustive) :

  • bootloader installation have timestamps
  • SQLite Nix store database contain registration times
  • /etc/shadow is in a non-deterministic order

A deterministic flag is available for best efforts determinism.

Usage

To produce a Nix-store only image: 

let
  pkgs = import <nixpkgs> {};
  lib = pkgs.lib;
  make-disk-image = import <nixpkgs/nixos/lib/make-disk-image.nix>;
in
  make-disk-image {
    inherit pkgs lib;
    config = {};
    additionalPaths = [ ];
    format = "qcow2";
    onlyNixStore = true;
    partitionTableType = "none";
    installBootLoader = false;
    touchEFIVars = false;
    diskSize = "auto";
    additionalSpace = "0M"; # Defaults to 512M.
    copyChannel = false;
  }

Some arguments can be left out, they are shown explicitly for the sake of the example.

Building this derivation will provide a QCOW2 disk image containing only the Nix store and its registration information.

To produce a NixOS installation image disk with UEFI and bootloader installed: 

let
  pkgs = import <nixpkgs> {};
  lib = pkgs.lib;
  make-disk-image = import <nixpkgs/nixos/lib/make-disk-image.nix>;
  evalConfig = import <nixpkgs/nixos/lib/eval-config.nix>;
in
  make-disk-image {
    inherit pkgs lib;
    inherit (evalConfig {
      modules = [
        {
          fileSystems."/" = { device = "/dev/vda"; fsType = "ext4"; autoFormat = true; };
          boot.grub.device = "/dev/vda";
        }
      ];
    }) config;
    format = "qcow2";
    onlyNixStore = false;
    partitionTableType = "legacy+gpt";
    installBootLoader = true;
    touchEFIVars = true;
    diskSize = "auto";
    additionalSpace = "0M"; # Defaults to 512M.
    copyChannel = false;
    memSize = 2048; # Qemu VM memory size in megabytes. Defaults to 1024M.
  }