nixos/zfs: add trim service

Introduces a trim timer similar to the fstrim service.
According to zpool(8) for consumer hardware periodic manual TRIM
is preferred over automatic TRIM that ZFS implements.
The period of one week is based on recommendations of fstrim.

nixos/modules/tasks/filesystems/zfs.nix

@@ -1,8 +1,6 @@
 { config, lib, pkgs, utils, ... }:
 #
-# todo:
-#   - crontab for scrubs, etc
-#   - zfs tunables
+# TODO: zfs tunables
 
 with utils;
 with lib;
@@ -13,6 +11,7 @@ let
   cfgSnapshots = config.services.zfs.autoSnapshot;
   cfgSnapFlags = cfgSnapshots.flags;
   cfgScrub = config.services.zfs.autoScrub;
+  cfgTrim = config.services.zfs.trim;
 
   inInitrd = any (fs: fs == "zfs") config.boot.initrd.supportedFilesystems;
   inSystem = any (fs: fs == "zfs") config.boot.supportedFilesystems;
@@ -268,14 +267,26 @@ in
       };
     };
 
-    services.zfs.autoScrub = {
-      enable = mkOption {
-        default = false;
-        type = types.bool;
+    services.zfs.trim = {
+      enable = mkEnableOption "Enables periodic TRIM on all ZFS pools.";
+
+      interval = mkOption {
+        default = "weekly";
+        type = types.str;
+        example = "daily";
         description = ''
-          Enables periodic scrubbing of ZFS pools.
+          How often we run trim. For most desktop and server systems
+          a sufficient trimming frequency is once a week.
+
+          The format is described in
+          <citerefentry><refentrytitle>systemd.time</refentrytitle>
+          <manvolnum>7</manvolnum></citerefentry>.
         '';
       };
+    };
+
+    services.zfs.autoScrub = {
+      enable = mkEnableOption "Enables periodic scrubbing of ZFS pools.";
 
       interval = mkOption {
         default = "Sun, 02:00";
@@ -535,5 +546,17 @@ in
         };
       };
     })
+
+    (mkIf cfgTrim.enable {
+      systemd.services.zpool-trim = {
+        description = "ZFS pools trim";
+        after = [ "zfs-import.target" ];
+        path = [ packages.zfsUser ];
+        startAt = cfgTrim.interval;
+        script = ''
+          zpool list -H -o name | xargs -n1 zpool trim
+        '';
+      };
+    })
   ];
 }