{ ... }: {
  services.btrbk = {
    instances = {
      # local snapshots:
      # - create hourly
      # - keep hourly for 4 days and daily forever
      home_local = {
        onCalendar = "hourly";
        settings = {
          snapshot_preserve = "96h *d";
          snapshot_preserve_min = "latest";
          volume."/" = {
            snapshot_dir = "/home/.btrbk";
            subvolume.home = {
              snapshot_create = "always";
            };
          };
        };
      };

      # backup snapshots to backup drive
      # - keep all hourly and the last 14 days of daily snapshots on the local disk
      # - keep 14 daily, 8 weekly and all monthly snapshots on the backup drive
      # - don't create new snapshots
      # - TODO: trigger on mount
      home_external = {
        # disable systemd timer
        onCalendar = null;
        settings = {
          # 14 * 24 = 336
          snapshot_preserve = "336h 14d";
          snapshot_preserve_min = "latest";

          target_preserve = "14d 8w *m";
          target_preserve_min = "2d";

          volume."/" = {
            snapshot_dir = "/home/.btrbk";
            subvolume.home = {
              snapshot_create = "no";
            };
            target = "/run/media/me/backup/morpheus";
          };
        };
      };
    };
  };
}