NixOS-Config/hosts/axolotl/boot/default.nix

{ pkgs, ... }:
{
  boot = {
    kernelPackages = pkgs.linuxPackages_latest;
    kernel.sysctl."vm.max_map_count" = 2147483642;

    initrd = {
      systemd = {
        enable = true;
        services.root-reset = {
          description = "Reset BTRFS root and snapshot last boot";
          wantedBy = [ "initrd.target" ];
          after = [ "dev-nixos-root.device" ];
          before = [ "sysroot.mount" ];
          unitConfig.DefaultDependencies = "no";
          serviceConfig.Type = "oneshot";
          script = ''
            mkdir -p /mnt
            mount /dev/nixos/root /mnt

            if [[ -e /mnt/@prev ]]; then
              btrfs subvolume delete /mnt/@prev
            fi

            btrfs subvolume snapshot /mnt/@ /mnt/@prev

            btrfs subvolume list -o /mnt/@ | cut -f9 -d' ' | while read subvolume; do
              btrfs subvolume delete "/mnt/$subvolume"
            done

            btrfs subvolume delete /mnt/@
            btrfs subvolume create /mnt/@

            umount /mnt
          '';
        };
      };
    };
  };
}