limit arc cache size to 5% of ram

modules/default.nix

@@ -15,6 +15,7 @@
     system/ssh.nix
     system/gaming.nix
     system/nix.nix
+    system/zfs.nix
 
     desktop/plasma.nix
     desktop/touchpad.nix

modules/system/zfs.nix

@@ -0,0 +1,37 @@
+{ config, lib, pkgs, modulesPath, ... }:
+
+{
+  config = lib.mkIf config.boot.zfs.enabled {
+    # Set ARC max to 5% of physical RAM at boot
+    systemd.services."zfs-arc-limit" = {
+      description = "Set ZFS ARC max to 5% of physical RAM";
+      # Ensure the module is loaded before we write to /sys
+      after = [ "systemd-modules-load.service" ];
+      # Run early, but it’s fine if ZFS has already imported; the limit still applies
+      wantedBy = [ "multi-user.target" ];
+      path = [ pkgs.gawk ];
+      script = ''
+        set -euo pipefail
+        # Total RAM in kB
+        mem_kb=$(awk '/MemTotal:/ {print $2}' /proc/meminfo)
+        echo "DEBUG: Total RAM: $mem_kb kB"
+        # 5%, in bytes
+        arc_max_bytes=$(( mem_kb * 1024 / 100 * 5 ))
+        echo "DEBUG: Setting ZFS ARC max to: $arc_max_bytes bytes"
+        param="/sys/module/zfs/parameters/zfs_arc_max"
+        if [ -w "$param" ]; then
+        echo "DEBUG: Writing to $param"
+        echo "$arc_max_bytes" > "$param"
+        echo "DEBUG: ZFS ARC max successfully set"
+        else
+        echo "WARN: $param not writable; is the zfs module loaded?" >&2
+        exit 0
+        fi
+      '';
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+      };
+    };
+  };
+}