Merge pull request #222205 from NickCao/zram-writeback-device

nixos/zram: add writebackDevice option and corresponding test

nixos/modules/config/zram.nix

@@ -82,12 +82,30 @@ in
           {command}`cat /sys/class/block/zram*/comp_algorithm`
         '';
       };
+
+      writebackDevice = lib.mkOption {
+        default = null;
+        example = "/dev/zvol/tarta-zoot/swap-writeback";
+        type = lib.types.nullOr lib.types.path;
+        description = lib.mdDoc ''
+          Write incompressible pages to this device,
+          as there's no gain from keeping them in RAM.
+        '';
+      };
     };
 
   };
 
   config = lib.mkIf cfg.enable {
 
+    assertions = [
+      {
+        assertion = cfg.writebackDevice == null || cfg.swapDevices <= 1;
+        message = "A single writeback device cannot be shared among multiple zram devices";
+      }
+    ];
+
+
     system.requiredKernelConfig = with config.lib.kernelConfig; [
       (isModule "ZRAM")
     ];
@@ -112,6 +130,8 @@ in
                 zram-size = if cfg.memoryMax != null then "min(${size}, ${toString cfg.memoryMax} / 1024 / 1024)" else size;
                 compression-algorithm = cfg.algorithm;
                 swap-priority = cfg.priority;
+              } // lib.optionalAttrs (cfg.writebackDevice != null) {
+                writeback-device = cfg.writebackDevice;
               };
           })
           devices));