nixConfig/nixos/modules/impermanence.nix

{lib, config, ...} :

let 
  inherit (lib)
    mkEnableOption
    mkIf
    mkOption
    types
    ;
  cfg = config.impermanence;
in

{
imports = [ ];

options.impermanence = {
  enable = mkEnableOption "impermanence";
  storagePath = mkOption {
    type = types.path;
    default = "/persist";
    description = "The path where persistent data is stored";
  };
  name = mkOption {
    type = types.str;
    default = "persist";
    description = "the name of the persistent data store";
  };
};

config = mkIf cfg.enable {
  environment.persistence.${cfg.name} = {
    persistentStoragePath = cfg.storagePath;
    directories = [
      "/var/log"
      "/var/lib/nixos"
    ];
    files = [
      "/etc/ssh/ssh_host_ed25519_key"
      "/etc/ssh/ssh_host_ed25519_key.pub"
      "/etc/ssh/ssh_host_rsa_key"
      "/etc/ssh/ssh_host_rsa_key.pub"
    ];
  };
  environment.etc.machine-id.source = "${cfg.storagePath}/machine-id";
};

}