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modules/nixos/linux/gc.nix

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+# System-level Nix garbage collection + store optimisation.
+#
+# The home-manager `nix.gc` (modules/home/nix/gc.nix) runs as the user, so it
+# can prune the *user* profile and collect the store, but it can never touch
+# the root-owned `system-*` profile generations. Without a system-level GC,
+# those pile up unbounded — pureintent accumulated ~480 system generations
+# (~4 months of daily rebuilds), each pinning a full closure and keeping old
+# kernels/packages from ever being collected. This prunes them; auto-optimise
+# hardlinks identical files across the closures that remain.
+{ ... }:
+{
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 30d";
+  };
+  nix.settings.auto-optimise-store = true;
+}