full rewrite, first commit

modules/nixos/services/btrfs/default.nix

@@ -0,0 +1,24 @@
+{
+  lib,
+  config,
+  pkgs,
+  namespace,
+  ...
+}:
+with lib;
+with lib.${namespace};
+let
+  cfg = config.${namespace}.services.btrfs;
+in
+{
+  options.${namespace}.services.btrfs = {
+    enable = mkBoolOpt true "BTRFS";
+  };
+
+  config = mkIf cfg.enable {
+    services.btrfs.autoScrub.enable = true;
+    services.fstrim.enable = true;
+
+    environment.systemPackages = with pkgs; [ btrfs-progs ];
+  };
+}