refactor: split "real" modules and "config" modules

config/optional/zfs.nix

@@ -0,0 +1,45 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}: {
+  boot.supportedFilesystems = ["zfs"];
+  boot.kernelPackages = config.boot.zfs.package.latestCompatibleLinuxPackages;
+
+  # The root pool should never be imported forcefully.
+  # Failure to import is important to notice!
+  boot.zfs.forceImportRoot = false;
+
+  environment.systemPackages = with pkgs; [zfs];
+
+  services.zfs = {
+    autoScrub = {
+      enable = true;
+      interval = "weekly";
+    };
+    trim = {
+      enable = true;
+      interval = "weekly";
+    };
+  };
+
+  services.telegraf.extraConfig.inputs = lib.mkIf config.services.telegraf.enable {
+    zfs.poolMetrics = true;
+  };
+
+  # TODO remove once this is upstreamed
+  boot.initrd.systemd.services."zfs-import-rpool".after = ["cryptsetup.target"];
+
+  # After importing the rpool, rollback the root system to be empty.
+  boot.initrd.systemd.services.impermanence-root = {
+    wantedBy = ["initrd.target"];
+    after = ["zfs-import-rpool.service"];
+    before = ["sysroot.mount"];
+    unitConfig.DefaultDependencies = "no";
+    serviceConfig = {
+      Type = "oneshot";
+      ExecStart = "${pkgs.zfs}/bin/zfs rollback -r rpool/local/root@blank";
+    };
+  };
+}