diff --git a/pageserver/src/tenant.rs b/pageserver/src/tenant.rs
index 5a36da938325..d556f7233545 100644
--- a/pageserver/src/tenant.rs
+++ b/pageserver/src/tenant.rs
@@ -6227,6 +6227,7 @@ mod tests {
         let cancel = CancellationToken::new();
 
         let base_key = Key::from_hex("620000000033333333444444445500000000").unwrap();
+        assert_eq!(base_key.field1, AUX_KEY_PREFIX); // in case someone accidentally changed the prefix...
         let mut test_key = base_key;
         let mut lsn = Lsn(0x10);
 
diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index bdaa55c880f8..28627e79117c 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -427,9 +427,12 @@ pub struct Timeline {
     /// Indicate whether aux file v2 storage is enabled.
     pub(crate) last_aux_file_policy: AtomicAuxFilePolicy,
 
-    #[cfg(test)]
     /// Some test cases directly place keys into the timeline without actually modifying the directory
-    /// keys (i.e., DB_DIR). Test cases will put such keyspaces here.
+    /// keys (i.e., DB_DIR). The test cases creating such keys will put the keyspaces here, so that
+    /// these keys won't get garbage-collected during compaction/GC. This field only modifies the dense
+    /// keyspace return value of `collect_keyspace`. For sparse keyspaces, use AUX keys for testing, and
+    /// in the future, add `extra_test_sparse_keyspace` if necessary.
+    #[cfg(test)]
     pub(crate) extra_test_dense_keyspace: ArcSwap<KeySpace>,
 }