initial test config

nixos/modules/impermanence.nix

@@ -0,0 +1,48 @@
+{
+  lib,
+  config,
+  ...
+}:
+let
+  inherit (lib)
+    mkEnableOption
+    mkIf
+    mkOption
+    types
+    ;
+  cfg = config.impermanence;
+in
+{
+  imports = [ ];
+
+  options.impermanence = {
+    enable = mkEnableOption "impermanence";
+    storagePath = mkOption {
+      type = types.path;
+      default = "/persist";
+      description = "The path where persistent data is stored";
+    };
+    name = mkOption {
+      type = types.str;
+      default = "persist";
+      description = "the name of the persistent data store";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.persistence.${cfg.name} = {
+      persistentStoragePath = cfg.storagePath;
+      directories = [
+        "/var/log"
+        "/var/lib/nixos"
+      ];
+      files = [
+        "/etc/ssh/ssh_host_ed25519_key"
+        "/etc/ssh/ssh_host_ed25519_key.pub"
+        "/etc/ssh/ssh_host_rsa_key"
+        "/etc/ssh/ssh_host_rsa_key.pub"
+      ];
+    };
+    environment.etc.machine-id.source = "${cfg.storagePath}/machine-id";
+  };
+}