NixOS: switch-root to tmpfs

0-nixos-tmpfs.md

Build a NixOS config, copy it to a tmpfs, and enter it, shutting the running userspace (but not the kernel) down. This can be useful e.g. for repartitioning the system or switching it to a new zfs pool, without the need for extra installation media, and is a little faster than most other volatile system methods (e.g. kexec, classic USB or CD installer), since the store is in RAM and uncompressed right from the get-go. It also copies the SSH host keys from the building system to maintain the chain of trust in a remote system.

nixos-tmpfs-change.sh

#!/usr/bin/env bash
set -exuo pipefail

mkdir -p /tmp/new-root-tmpfs
findmnt /tmp/new-root-tmpfs || mount -t tmpfs tmpfs /tmp/new-root-tmpfs
system=$(nix-build '<nixpkgs/nixos>' --arg configuration '{ pkgs, modulesPath, lib, ... }: {
  imports = [
    "${modulesPath}/installer/cd-dvd/installation-cd-graphical-calamares-gnome.nix"
  ];

  # use the latest Linux kernel
  boot.kernelPackages = pkgs.linuxPackages_latest;
fileSystems."/" = {
      device = "none";
      fsType = "tmpfs";
  };

  boot.loader.grub.enable = false;
  # Needed for https://github.com/NixOS/nixpkgs/issues/58959
  boot.supportedFilesystems = lib.mkForce [ "btrfs" "reiserfs" "vfat" "f2fs" "xfs" "ntfs" "cifs" ];
} ' -A config.system.build.toplevel)

nix copy $system --to /tmp/new-root-tmpfs --no-require-sigs --extra-experimental-features nix-command
ln -s $system /tmp/new-root-tmpfs/system
mkdir -p /tmp/new-root-tmpfs/etc

[[ $(read -p "Switch into the tmpfs system? ") = y ]] && systemctl switch-root /tmp/new-root-tmpfs /system/init