{ 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)"

  # TODO: Maybe the following is not necessary if using hd-idle anyways (!)
  # TODO: Mount/Unmount backup drive
  #   Example1:
  #     - https://github.com/oxalica/nixos-config/blob/1b18628afda9cecca0bc8c348b91e3ae6ab56905/nixos/invar/btrbk.nix#L55
  #     - https://github.com/oxalica/nixos-config/blob/1b18628afda9cecca0bc8c348b91e3ae6ab56905/nixos/invar/btrbk.nix#L64
  #   Example2:
  #     - https://github.com/emmanuelrosa/erosanix/blob/master/modules/btrbk.nix#L35-L42C7

  # Spin down (unmounted/inactive) HDD disk.
  # Inspired by https://www.reddit.com/r/NixOS/comments/751i5t/comment/do3f3l7/
  #
#  environment.systemPackages = with pkgs; [
#    hd-idle
#  ];
#  systemd.services.hd-idle = {
#    description = "Spin down inactive HDD";
#    wantedBy = [ "multi-user.target" ];
#    serviceConfig = {
#      Type = "simple";
#      # Options:
#      # -a <disk>: Select disk for subsequent parameters.
#      # -i 180: Spin down after 2 minutes (180 seconds) of inactivity.
#      ExecStart = "${pkgs.hd-idle}/bin/hd-idle -a /dev/disk/by-id/XXX-XXX-XXX -i 180"; # TODO make disk id configurable
#    };
#  };

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

#      # This is the configuration equivalent to
#      # README.md section "Testing on Local Host".
#      "testing-on-local-host" = {
#        # The timer is disabled. The service has to be invoked manually.
#        onCalendar = null;
#        #onCalendar = "hourly";
#        #onCalendar = "*:0/15"; # Every 15min
#        # Configuration options for btrbk. Nested attrsets translate to subsections.
#        settings = {
#          timestamp_format = "long";
#          stream_compress = "lz4";
#
#          snapshot_preserve_min = "2d";
#          snapshot_preserve = "24h 7d 4w 6m";
#
#          target_preserve_min = "no";
#          target_preserve = "7d 4w 6m";
#
#          volume."/mnt/top-level" = {
#            snapshot_dir = "@snap";
#            target = "/mnt/usb-top-level/@snap";
#            subvolume = "@foo";
#          };
#        };
#      };

      "local-snapshots" = {
        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."/mnt/data/data" = {
            snapshot_dir = "/mnt/data/snap";
            subvolume = {
              "arch.p1st.de" = {};
              "blogger.privacy1st.de" = {};
              "changedetection.p1st.de" = {};
              "cloud.media-kollektiv.eu" = {};
              "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" = {};
            };
          };
        };
      };

    };
  };
}