Add the hostid option. Refactor code a bit better for initrd(untested) and system operation.

modules/tasks/filesystems/zfs.nix

@@ -1,41 +1,68 @@
 { config, pkgs, ... }:
+#
+# todo:
+#   - crontab for scrubs, etc
+#   - zfs tunables
+#   - /etc/zfs/zpool.cache handling
+
 
 with pkgs.lib;
 
 let
 
+  cfgSpl = config.environment.spl;
   inInitrd = any (fs: fs == "zfs") config.boot.initrd.supportedFilesystems;
+  inSystem = any (fs: fs == "zfs") config.boot.supportedFilesystems;
   kernel = config.boot.kernelPackages;
 
 in
 
 {
-  ###### implementation
-
-  config = mkIf (any (fs: fs == "zfs") config.boot.supportedFilesystems) {
 
-    boot.kernelModules = [ "spl" "zfs" ] ;
-    boot.extraModulePackages = [ kernel.zfs kernel.spl ];
+  ###### interface
+  
+  options = { 
+    environment.spl.hostid = mkOption { 
+      default = "";
+      example = "0xdeadbeef";
+      description = ''
+        ZFS uses a system's hostid to determine if a storage pool (zpool) is
+        native to this system, and should thus be imported automatically.
+        Unfortunately, this hostid can change under linux from boot to boot (by
+        changing network adapaters, for instance). Specify a unique 32 bit hostid in
+        hex here for zfs to prevent getting a random hostid between boots and having to
+        manually import pools.
+      '';
+    };
+  };
 
-    boot.initrd.kernelModules = mkIf inInitrd [ "spl" "zfs" ] ;
+  ###### implementation
 
-    boot.initrd.extraUtilsCommands = mkIf inInitrd
-      ''
-        cp -v ${kernel.zfs}/sbin/zfs $out/sbin
-        cp -v ${kernel.zfs}/sbin/zdb $out/sbin
-        cp -v ${kernel.zfs}/sbin/zpool $out/sbin
-      '';
+  config = mkIf ( inInitrd || inSystem ) {
 
-    boot.initrd.postDeviceCommands = mkIf inInitrd
-      ''
-        zpool import -f -a -d /dev
-        zfs mount -a
-      '';
+    boot = { 
+      kernelModules = [ "spl" "zfs" ] ;
+      extraModulePackages = [ kernel.zfs kernel.spl ];
+      extraModprobeConfig = mkIf (cfgSpl.hostid != "") "options spl spl_hostid=${cfgSpl.hostid}";
+    };
 
-    system.fsPackages = [ kernel.zfs ];
+    boot.initrd = mkIf inInitrd { 
+      kernelModules = [ "spl" "zfs" ] ;
+      extraUtilsCommands =
+        ''
+          cp -v ${kernel.zfs}/sbin/zfs $out/sbin
+          cp -v ${kernel.zfs}/sbin/zdb $out/sbin
+          cp -v ${kernel.zfs}/sbin/zpool $out/sbin
+        '';
+      postDeviceCommands =
+        ''
+          zpool import -f -a -d /dev
+          zfs mount -a
+        '';
+    };
 
+    system.fsPackages = [ kernel.zfs ];                  # XXX: needed? zfs doesn't have a fsck
     environment.systemPackages = [ kernel.zfs ];
-
-    services.udev.packages = [ kernel.zfs ];
+    services.udev.packages = [ kernel.zfs ];             # to hook zvol naming, etc. 
   };
 }