refactor(nixos): make ZFS configuration optional

- move ZFS and snapshot manager settings out of the global base
- group snapshot files under common/optional/zfs
- enable the ZFS module on existing ZFS hosts
- make the global tmpfs setting overridable

common/global/default.nix

@@ -17,16 +17,11 @@
     ./nix.nix
     ./programs.nix
     ./ssh.nix
-    ./snapshot_manager.nix
   ];
 
   boot = {
-    tmp.useTmpfs = true;
+    tmp.useTmpfs = lib.mkDefault true;
     kernelPackages = lib.mkDefault pkgs.linuxPackages_6_12;
-    zfs = {
-      package = lib.mkDefault pkgs.zfs_2_4;
-      forceImportRoot = lib.mkDefault false;
-    };
   };
 
   hardware.enableRedistributableFirmware = true;
@@ -50,16 +45,6 @@
 
     # firmware update
     fwupd.enable = true;
-
-    snapshot_manager = {
-      enable = lib.mkDefault true;
-      PYTHONPATH = "${inputs.self}/";
-    };
-
-    zfs = {
-      trim.enable = lib.mkDefault true;
-      autoScrub.enable = lib.mkDefault true;
-    };
   };
 
   powerManagement.powertop.enable = lib.mkDefault true;

common/optional/zfs/default.nix

@@ -0,0 +1,26 @@
+{
+  inputs,
+  lib,
+  pkgs,
+  ...
+}:
+{
+  imports = [ ./snapshot.nix ];
+
+  boot.zfs = {
+    package = lib.mkDefault pkgs.zfs_2_4;
+    forceImportRoot = lib.mkDefault false;
+  };
+
+  services = {
+    snapshot_manager = {
+      enable = lib.mkDefault true;
+      PYTHONPATH = "${inputs.self}/";
+    };
+
+    zfs = {
+      trim.enable = lib.mkDefault true;
+      autoScrub.enable = lib.mkDefault true;
+    };
+  };
+}

systems/bob/default.nix

@@ -6,15 +6,16 @@
     "${inputs.self}/users/steve"
     "${inputs.self}/common/global"
     "${inputs.self}/common/optional/docker.nix"
-    "${inputs.self}/common/optional/scanner.nix"
     "${inputs.self}/common/optional/monitoring-agent.nix"
+    "${inputs.self}/common/optional/nvidia.nix"
+    "${inputs.self}/common/optional/scanner.nix"
     "${inputs.self}/common/optional/steam.nix"
     "${inputs.self}/common/optional/syncthing_base.nix"
     "${inputs.self}/common/optional/systemd-boot.nix"
     "${inputs.self}/common/optional/update.nix"
     "${inputs.self}/common/optional/yubikey.nix"
     "${inputs.self}/common/optional/zerotier.nix"
-    "${inputs.self}/common/optional/nvidia.nix"
+    "${inputs.self}/common/optional/zfs"
     ./hardware.nix
     ./syncthing.nix
     ./llms.nix

systems/brain/default.nix

@@ -9,6 +9,7 @@
     "${inputs.self}/common/optional/systemd-boot.nix"
     "${inputs.self}/common/optional/update.nix"
     "${inputs.self}/common/optional/zerotier.nix"
+    "${inputs.self}/common/optional/zfs"
     ./docker
     ./hardware.nix
     ./programs.nix

systems/jeeves/default.nix

@@ -15,6 +15,7 @@ in
     "${inputs.self}/common/optional/syncthing_base.nix"
     "${inputs.self}/common/optional/update.nix"
     "${inputs.self}/common/optional/zerotier.nix"
+    "${inputs.self}/common/optional/zfs"
     ./monitoring
     ./docker
     ./services

systems/rhapsody-in-green/default.nix

@@ -10,6 +10,7 @@
     "${inputs.self}/common/optional/systemd-boot.nix"
     "${inputs.self}/common/optional/yubikey.nix"
     "${inputs.self}/common/optional/zerotier.nix"
+    "${inputs.self}/common/optional/zfs"
     ./hardware.nix
     ./open_webui.nix
     ./programs.nix