Group shared system settings

system/shared/nix/gc.nix

@@ -0,0 +1,27 @@
+_: {
+  nix = {
+    settings = {
+      # nix often takes up a lot of space, with /nix/store growing beyond reasonable sizes
+      # this turns on automatic optimisation of the nix store that will run during every build
+      # (alternatively, you can do this manually with `nix-store --optimise`)
+      auto-optimise-store = true;
+    };
+
+    # Enable automatic garbage collection, deleting entries older than 7 days
+    # you can also run this manually with `nix-store --gc --delete-older-than 7d`.
+    # If a result still exists in the file system, all the dependencies used to build
+    # it will be kept.
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 7d";
+    };
+
+    # Also run garbage colleciton whenever there is not enough space left,
+    # freeing up to 1 GiB whenever there is less than 512MiB left.
+    extraOptions = ''
+      min-free = ${toString (512 * 1024 * 1024)}
+      max-free = ${toString (1024 * 1024 * 1024)}
+    '';
+  };
+}