{ config, pkgs, ... }:
{
  # For each instance, a service and timer will be created:
  #   systemctl list-unit-files | grep btrbk
  #   #=> btrbk-<instance-name>.service
  #   #=> btrbk-<instance-name>.timer
  #
  # If onCalendar is set to `null`, one can manually execute btrbk with
  #   sudo systemctl start btrbk-<instance-name>.service

  # Print generated systemd unit file
  #   cat "$(systemctl show -P FragmentPath btrbk-<instance-name>.service)"
  #   #=> ExecStart=/nix/store/53nvbl1c0w14524j7v3fpn9py31yi2hb-btrbk-0.32.6/bin/btrbk -c /etc/btrbk/local-backup.conf run

  services.btrbk = {
    extraPackages = [ pkgs.lz4 ];
    # Lowest scheduling priority.
    niceness = 19;
    # Set of btrbk instances. The instance named btrbk is the default one.
    instances = {

      "local-snapshot" = {
        onCalendar = "hourly";
        #onCalendar = "*:0/15"; # Every 15min

        settings = {
          timestamp_format = "long";
          #stream_compress =  "lz4";

          snapshot_preserve_min = "2d";
          snapshot_preserve =     "24h 7d 4w 6m";

          volume."/jc-data" = {
            snapshot_dir = "/snap";
            subvolume = {
              "arch.p1st.de" = {};
              "blogger.privacy1st.de" = {};
              "changedetection.p1st.de" = {};
              "cloud.privacy1st.de" = {};
              "git.privacy1st.de" = {};
              "mastodon-toot-follower.privacy1st.de" = {};
              "money.p1st.de" = {};
              "music.privacy1st.de" = {};
              "paste.p1st.de" = {};
              "proxy" = {};
              "recipe.privacy1st.de" = {};
              "traggo.privacy1st.de" = {};
            };
          };
          volume."/mnt/data/jc-data" = {
            snapshot_dir = "/mnt/data/snap";
            subvolume = {
              "cloud.privacy1st.de" = {};
              # MediaKollektiv: 796 GiB
              "cloud.media-kollektiv.eu" = {};
            };
          };
        };
      };

      "local-backup" = {
        # Run daily, 5 minutes after `local-snapshot` 0'o'clock timer.
        onCalendar = "00:05";

        settings = {
          timestamp_format = "long";
          #stream_compress =  "lz4";

          # Create backups.
          target_preserve_min = "no";
          target_preserve =     "7d 4w 6m";

          # Don't create or delete snapshots.
          snapshot_preserve_min = "all";
          snapshot_create       = "no";

          volume."/jc-data" = {
            snapshot_dir = "/snap";
            target = "/mnt/backup/snap";
            subvolume = {
              "arch.p1st.de" = {};
              "blogger.privacy1st.de" = {};
              "changedetection.p1st.de" = {};
              "cloud.privacy1st.de" = {};
              "git.privacy1st.de" = {};
              "mastodon-toot-follower.privacy1st.de" = {};
              "money.p1st.de" = {};
              "music.privacy1st.de" = {};
              "paste.p1st.de" = {};
              "proxy" = {};
              "recipe.privacy1st.de" = {};
              "traggo.privacy1st.de" = {};
            };
          };
          volume."/mnt/data/jc-data" = {
            snapshot_dir = "/mnt/data/snap";
            target = "/mnt/backup/snap2";
            subvolume = {
              "cloud.privacy1st.de" = {};
              # MediaKollektiv: 796 GiB
              "cloud.media-kollektiv.eu" = {};
            };
          };
        };
      };

    };
  };
}