HDFS-742. A down DataNode makes Balancer to hang on repeatingly asking NameNode its partial block list. Contributed by Mit Desai.

(cherry picked from commit 58db263e93)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/Dispatcher.java

@@ -788,6 +788,13 @@ private void dispatchBlocks() {
               + ", scheduledSize=" + getScheduledSize()
               + ", srcBlocks#=" + srcBlocks.size());
         }
+        // check if time is up or not
+        if (Time.monotonicNow() - startTime > MAX_ITERATION_TIME) {
+          LOG.info("Time up (max time=" + MAX_ITERATION_TIME/1000
+              + " seconds).  Skipping " + this);
+          isTimeUp = true;
+          continue;
+        }
         final PendingMove p = chooseNextMove();
         if (p != null) {
           // Reset no pending move counter
@@ -825,14 +832,6 @@ private void dispatchBlocks() {
           }
         }
 
-        // check if time is up or not
-        if (Time.monotonicNow() - startTime > MAX_ITERATION_TIME) {
-          LOG.info("Time up (max time=" + MAX_ITERATION_TIME/1000
-              + " seconds).  Skipping " + this);
-          isTimeUp = true;
-          continue;
-        }
-
         // Now we can not schedule any block to move and there are
         // no new blocks added to the source block list, so we wait.
         try {