nixos/image: add repart builder docs

nixos/modules/image/repart.md

@@ -0,0 +1,137 @@
+# 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
+```

nixos/modules/image/repart.nix

@@ -198,7 +198,10 @@ in
           image.raw
       '';
 
-    meta.maintainers = with lib.maintainers; [ nikstur ];
+    meta = {
+      maintainers = with lib.maintainers; [ nikstur ];
+      doc = ./repart.md;
+    };
 
   };
 }