Add impermanence config

options/system/default.nix

@@ -2,7 +2,9 @@
   inherit (lib) mkOption;
 in
 {
-  imports = [ ];
+  imports = [
+    ./impermanence.nix
+  ];
 
   options.myOptions.system = {
     hostname = mkOption {

options/system/impermanence.nix

@@ -0,0 +1,81 @@
+{ lib, config, ... }: with lib; let
+  inherit (lib) mkEnableOption mkOption literalExpression;
+
+  cfg = config.myOptions.system.impermanence;
+in
+{
+  options.myOptions.system.impermanence = {
+    root = {
+      enable = mkEnableOption ''
+        the Impermanence module for persisting important state directories.
+      '';
+
+      extraFiles = mkOption {
+        default = [];
+        example = literalExpression ''["/etc/nix/id_rsa"]'';
+        description = ''
+          Additional files in root to link to persistent storage.
+        '';
+      };
+
+      extraDirectories = mkOption {
+        default = [];
+        example = literalExpression ''["/etc/nix/id_rsa"]'';
+        description = ''
+          Additional directories in root to link to persistent storage.
+        '';
+      };
+
+      persistentMountPoint = mkOption {
+        default = "/persist";
+        description = ''
+          Path to a persistent directory (usually a mount point to a
+          standalone partition / subvolume), which will hold the persistent
+          system state files.
+        '';
+      };
+
+      autoBtrfsWipe = {
+        enable = mkOption {
+          default = true;
+          description = ''
+            Enable automatic wiping of the root BTRFS subvolume from initrd.
+
+            Generally, you will want to keep this enabled, as otherwise setting up
+            impermanence is pointless. However in case you're using a non-BTRFS
+            system, or you wish to set up a custom handling for this auto-wiping,
+            which the current handling doesn't support, disable this.
+          '';
+        };
+
+        devicePath = mkOption {
+          default = "/dev/mapper/cryptfs";
+          description = ''
+            Path to the BTRFS block device containing the subvolume to be wiped.
+
+            This device will be mounted from initrd.
+          '';
+        };
+
+        subvolumePath = mkOption {
+          default = "root";
+          description = ''
+            Path to the BTRFS subvolume to be wiped.
+
+            This is a relative path, starting from the BTRFS root.
+          '';
+        };
+
+        cleanSnapshotPath = mkOption {
+          default = "root-blank";
+          description = ''
+            Path to the BTRFS snapshot (subvolume) to be restore 
+            `myOptions.system.impermanence.root.autoWipe.btrfsSubvolume` to.
+
+            This should be a blank snapshot to achieve a complete wipe.
+          '';
+        };
+      };
+    };
+  };
+}