# UNDF: UNDF-2026-000000097
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/HeartbeatManager.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/HeartbeatManager.java
@@ -18,7 +18,9 @@ import java.util.ArrayList;
 import java.util.List;
+import java.util.HashSet;
+import java.util.Set;

 // ... (other imports unchanged)

@@ -447,8 +447,8 @@ class HeartbeatManager implements DatanodeStatistics {
     boolean allAlive = false;
     // Locate limited dead nodes.
-    List<DatanodeDescriptor> deadDatanodes = new ArrayList<>(
-        numOfDeadDatanodesRemove);
+    Set<DatanodeDescriptor> deadDatanodes = new HashSet<>(
+        numOfDeadDatanodesRemove * 2);
     // Locate limited failed storages that isn't on a dead node.
     List<DatanodeStorageInfo> failedStorages = new ArrayList<>(
         numOfDeadDatanodesRemove);
@@ -493,7 +493,7 @@ class HeartbeatManager implements DatanodeStatistics {
           if (failedStorages.size() < numOfDeadDatanodesRemove &&
               storageInfo.areBlocksOnFailedStorage() &&
-              !deadDatanodes.contains(d)) {
+              !deadDatanodes.contains(d)) {  // now O(1) via HashSet
             failedStorages.add(storageInfo);
           }
         }