HDFS-9790. HDFS Balancer should exit with a proper message if upgrade is not finalized (Contributed by Xiaobing Zhou)

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -2739,6 +2739,9 @@ Release 2.8.0 - UNRELEASED
 
     HDFS-9788. Incompatible tag renumbering in HeartbeatResponseProto. (wang)
 
+    HDFS-9790. HDFS Balancer should exit with a proper message if upgrade is
+    not finalized. (Xiaobing Zhou via Arpit Agarwal)
+
 Release 2.7.3 - UNRELEASED
 
   INCOMPATIBLE CHANGES

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

@@ -586,6 +586,8 @@ public class Balancer {
       // Should not run the balancer during an unfinalized upgrade, since moved
       // blocks are not deleted on the source datanode.
       if (!runDuringUpgrade && nnc.isUpgrading()) {
+        System.err.println("Balancer exiting as upgrade is not finalized, "
+            + "please finalize the HDFS upgrade before running the balancer.");
         return newResult(ExitStatus.UNFINALIZED_UPGRADE, bytesLeftToMove, -1);
       }