nix-git/modules/btrbk/default.nix

# 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

{ config, pkgs, ... }:
{
  imports = [
    ./snapshot.nix
    ./backup.nix
  ];

  # Compression
  #
  # `btrbk` can be configured to either
  # - compress the BTRFS data stream itself
  # - or to enable SSH compression
  #
  # The latter supports only ZLIB (LZ77) compression, see https://www.ietf.org/rfc/rfc4253.txt
  # Thus, we choose the first option instead.
  #
  # lz4 is a good option, but zstd-9 seems to be better in the mean time.
  #
  # Install the compression package so that other hosts can pull compressed snapshots from us.
  environment.systemPackages = [
    pkgs.zstd
  ];

  services.btrbk = {
    # Lowest scheduling priority.
    niceness = 19;
    # The `instances` option is set by `./snapshot.nix` and `./backup.nix`.
  };
}