Logging stale datanode information. Contributed by Karthik Palanisamy.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/HeartbeatManager.java

@@ -379,6 +379,9 @@ class HeartbeatManager implements DatanodeStatistics {
             dead = d;
           }
           if (d.isStale(dm.getStaleInterval())) {
+            LOG.warn(String.format("Stale datanode {}."
+                    + " No heartbeat received since last {} milliseconds"),
+                    d.getName(), dm.getStaleInterval());
             numOfStaleNodes++;
           }
           DatanodeStorageInfo[] storageInfos = d.getStorageInfos();