HDFS-14171. Performance improvement in Tailing EditLog. Contributed by Kenneth Yang.

(cherry picked from commit e9a005dfcf4d2249235aeff4da85b321dddc1ca9)
(cherry picked from commit c48640ffbd3852b75f898325ae0b3ea0ba864578)
(cherry picked from commit f24684ca9cd70326e24376a33cda937c98295af0)
(cherry picked from commit 054594c6145711c60f3cb848f08e515c4179f0c4)

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

@@ -541,7 +541,12 @@ class BlockManagerSafeMode {
    */
   private boolean areThresholdsMet() {
     assert namesystem.hasWriteLock();
-    int datanodeNum = blockManager.getDatanodeManager().getNumLiveDataNodes();
+    // Calculating the number of live datanodes is time-consuming
+    // in large clusters. Skip it when datanodeThreshold is zero.
+    int datanodeNum = 0;
+    if (datanodeThreshold > 0) {
+      datanodeNum = blockManager.getDatanodeManager().getNumLiveDataNodes();
+    }
     synchronized (this) {
       return blockSafe >= blockThreshold && datanodeNum >= datanodeThreshold;
     }