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# Building Images via `systemd-repart` {#sec-image-repart}
You can build disk images in NixOS with the `image.repart` option provided by
the module [image/repart.nix][]. This module uses `systemd-repart` to build the
images and exposes it's entire interface via the `repartConfig` option.
[image/repart.nix]: https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/image/repart.nix
An example of how to build an image:
```nix
{ config, modulesPath, ... }: {
imports = [ "${modulesPath}/image/repart.nix" ];
image.repart = {
name = "image";
partitions = {
"esp" = {
contents = {
...
};
repartConfig = {
Type = "esp";
...
};
};
"root" = {
storePaths = [ config.system.build.toplevel ];
repartConfig = {
Type = "root";
Label = "nixos";
...
};
};
};
};
}
```
## Nix Store Partition {#sec-image-repart-store-partition}
You can define a partition that only contains the Nix store and then mount it
under `/nix/store`. Because the `/nix/store` part of the paths is already
determined by the mount point, you have to set `stripNixStorePrefix = true;` so
that the prefix is stripped from the paths before copying them into the image.
```nix
fileSystems."/nix/store".device = "/dev/disk/by-partlabel/nix-store"
image.repart.partitions = {
"store" = {
storePaths = [ config.system.build.toplevel ];
stripNixStorePrefix = true;
repartConfig = {
Type = "linux-generic";
Label = "nix-store";
...
};
};
};
```
## Appliance Image {#sec-image-repart-appliance}
The `image/repart.nix` module can also be used to build self-contained [software
appliances][].
[software appliances]: https://en.wikipedia.org/wiki/Software_appliance
The generation based update mechanism of NixOS is not suited for appliances.
Updates of appliances are usually either performed by replacing the entire
image with a new one or by updating partitions via an A/B scheme. See the
[Chrome OS update process][chrome-os-update] for an example of how to achieve
this. The appliance image built in the following example does not contain a
`configuration.nix` and thus you will not be able to call `nixos-rebuild` from
this system.
[chrome-os-update]: https://chromium.googlesource.com/aosp/platform/system/update_engine/+/HEAD/README.md
```nix
let
pkgs = import <nixpkgs> { };
efiArch = pkgs.stdenv.hostPlatform.efiArch;
in
(pkgs.nixos [
({ config, lib, pkgs, modulesPath, ... }: {
imports = [ "${modulesPath}/image/repart.nix" ];
boot.loader.grub.enable = false;
fileSystems."/".device = "/dev/disk/by-label/nixos";
image.repart = {
name = "image";
partitions = {
"esp" = {
contents = {
"/EFI/BOOT/BOOT${lib.toUpper efiArch}.EFI".source =
"${pkgs.systemd}/lib/systemd/boot/efi/systemd-boot${efiArch}.efi";
"/loader/entries/nixos.conf".source = pkgs.writeText "nixos.conf" ''
title NixOS
linux /EFI/nixos/kernel.efi
initrd /EFI/nixos/initrd.efi
options init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams}
'';
"/EFI/nixos/kernel.efi".source =
"${config.boot.kernelPackages.kernel}/${config.system.boot.loader.kernelFile}";
"/EFI/nixos/initrd.efi".source =
"${config.system.build.initialRamdisk}/${config.system.boot.loader.initrdFile}";
};
repartConfig = {
Type = "esp";
Format = "vfat";
SizeMinBytes = "96M";
};
};
"root" = {
storePaths = [ config.system.build.toplevel ];
repartConfig = {
Type = "root";
Format = "ext4";
Label = "nixos";
Minimize = "guess";
};
};
};
};
})
]).image
```
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