This is a post of things I wanted to do in nixos but isn’t described anywhere. I had to read source code to figure these things out. By explaining here what is going on I make things easier for other people.

Nix custom image

It’s possible to bypass virtual box and make a bunch of different image formats directly, I used two files image.nix:

{ config, ... }:

let
  pkgs = import ./pin.nix { };
in

{ 
  system.build.image = import <nixpkgs/nixos/lib/make-disk-image.nix> {
      name = "nixos-vmdk-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}";
      format = "vpc";
      inherit pkgs config;
      lib = pkgs.lib;
      partitionTableType = "legacy";
      diskSize = 11 * 1024;
  };
  .... # remaining config, same as configuration.nix
}

and the disk.nix:

{ nixos ? <nixpkgs/nixos>
, system ? builtins.currentSystem
}:

let
  machine-configuration = import ./image.nix;
  machine = import nixos {
    inherit system;
    configuration = machine-configuration;
  };
in 
machine.config.system.build.image

If you run nix-build disk.nix you’ll get a VHD with the configuration from image.nix which is just a nixos standard configuraiton.nix.

Nixos-rebuild remote

We can use nixos-rebuild to do in place updates of a running system remotely. If your deployment is a single VM this is significantly easier than using nixops. I ended up with this make file:

IP="192.168.0.39"
deploy: 
    NIXOS_CONFIG=$(shell pwd)"/image.nix" nixos-rebuild switch --target-host root@$(IP)

You may also want to set --build-host, because by default it will build on target-host.

Install nix on running nix

For some reason my boot disk gets corrupted after switching a couple of times. A solution is just to never reboot, however you can also just fix this while running the system. Switching doesn’t do this apparently. I know this because it didn’t boot. This doesn’t matter because you can install your currently running system!

Mount root as /mnt. Format your boot partition and mount it on /mnt/boot. Then run nixos-install. For example:

mount # list everything mounted
mount /dev/nvme0n1p2 /mnt
umount /boot
mkfs.vfat -n boot /dev/nvme0n1p1
mount /dev/nvme0n1p1 /boot
mount /dev/nvme0n1p1 /mnt/boot