nixos/swapspace: init module

Signed-off-by: phanirithvij <phanirithvij2000@gmail.com>

Apply suggestions from code review

Co-authored-by: Luflosi <luflosi@luflosi.de>

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -183,6 +183,8 @@
 
 - [Fedimint](https://github.com/fedimint/fedimint), a module based system for building federated applications (Federated E-Cash Mint). Available as [services.fedimintd](#opt-services.fedimintd).
 
+- [Swapspace](https://github.com/Tookmund/Swapspace), a dynamic swap space manager, turns your unused free space into swap automatically. Available as [services.swapspace](#opt-services.swapspace.enable).
+
 ## Backward Incompatibilities {#sec-release-24.11-incompatibilities}
 
 - The `sound` options have been removed or renamed, as they had a lot of unintended side effects. See [below](#sec-release-24.11-migration-sound) for details.

nixos/modules/module-list.nix

@@ -1353,6 +1353,7 @@
   ./services/system/nscd.nix
   ./services/system/saslauthd.nix
   ./services/system/self-deploy.nix
+  ./services/system/swapspace.nix
   ./services/system/systembus-notify.nix
   ./services/system/systemd-lock-handler.nix
   ./services/system/uptimed.nix

nixos/modules/services/system/swapspace.nix

@@ -0,0 +1,118 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  cfg = config.services.swapspace;
+  inherit (lib)
+    types
+    mkOption
+    mkPackageOption
+    mkEnableOption
+    ;
+  configFile = pkgs.writeText "swapspace.conf" (lib.generators.toKeyValue { } cfg.settings);
+in
+{
+  options.services.swapspace = {
+    enable = mkEnableOption "Swapspace, a dynamic swap space manager";
+    package = mkPackageOption pkgs "swapspace" { };
+    extraArgs = mkOption {
+      type = types.str;
+      default = "";
+      example = "-P -v";
+      description = "Any extra arguments to pass to swapspace";
+    };
+    settings = mkOption {
+      type = types.submodule {
+        options = {
+          swappath = mkOption {
+            type = types.str;
+            default = "/var/lib/swapspace";
+            description = "Location where swapspace may create and delete swapfiles";
+          };
+          lower_freelimit = mkOption {
+            type = types.ints.between 0 99;
+            default = 20;
+            description = "Lower free-space threshold: if the percentage of free space drops below this number, additional swapspace is allocated";
+          };
+          upper_freelimit = mkOption {
+            type = types.ints.between 0 100;
+            default = 60;
+            description = "Upper free-space threshold: if the percentage of free space exceeds this number, swapspace will attempt to free up swapspace";
+          };
+          freetarget = mkOption {
+            type = types.ints.between 2 99;
+            default = 30;
+            description = ''
+              Percentage of free space swapspace should aim for when adding swapspace.
+              This should fall somewhere between lower_freelimit and upper_freelimit.
+            '';
+          };
+          min_swapsize = mkOption {
+            type = types.str;
+            default = "4m";
+            description = "Smallest allowed size for individual swapfiles";
+          };
+          max_swapsize = mkOption {
+            type = types.str;
+            default = "2t";
+            description = "Greatest allowed size for individual swapfiles";
+          };
+          cooldown = mkOption {
+            type = types.ints.unsigned;
+            default = 600;
+            description = ''
+              Duration (roughly in seconds) of the moratorium on swap allocation that is instated if disk space runs out, or the cooldown time after a new swapfile is successfully allocated before swapspace will consider deallocating swap space again.
+              The default cooldown period is about 10 minutes.
+            '';
+          };
+          buffer_elasticity = mkOption {
+            type = types.ints.between 0 100;
+            default = 30;
+            description = ''Percentage of buffer space considered to be "free"'';
+          };
+          cache_elasticity = mkOption {
+            type = types.ints.between 0 100;
+            default = 80;
+            description = ''Percentage of cache space considered to be "free"'';
+          };
+        };
+      };
+      default = { };
+      description = ''
+        Config file for swapspace.
+        See the options here: <https://github.com/Tookmund/Swapspace/blob/master/swapspace.conf>
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    systemd.tmpfiles.rules = [ "d '${cfg.settings.swappath}' 0700 - - - - " ];
+    environment.systemPackages = [ cfg.package ];
+    systemd.services.swapspace = {
+      after = [
+        "local-fs.target"
+        "swap.target"
+      ];
+      requires = [
+        "local-fs.target"
+        "swap.target"
+      ];
+      wantedBy = [ "multi-user.target" ];
+      documentation = [ "man:swapspace(8)" ];
+      description = "Swapspace, a dynamic swap space manager";
+      serviceConfig = {
+        ExecStart = "${lib.getExe cfg.package} -c ${configFile} ${cfg.extraArgs}";
+      };
+    };
+  };
+
+  meta = {
+    maintainers = with lib.maintainers; [
+      Luflosi
+      phanirithvij
+    ];
+  };
+}