diff --git a/src/libstore/include/nix/store/local-store.hh b/src/libstore/include/nix/store/local-store.hh
index 444d1b28f..1184be8ed 100644
--- a/src/libstore/include/nix/store/local-store.hh
+++ b/src/libstore/include/nix/store/local-store.hh
@@ -174,6 +174,10 @@ private:
         std::unique_ptr<PublicKeys> publicKeys;
     };
 
+    /**
+     * Mutable state. It's behind a `ref` to reduce false sharing
+     * between immutable and mutable fields.
+     */
     ref<Sync<State>> _state;
 
 public: