HDFS.7849. Update documentation for enabling a new feature in rolling upgrade ( Contributed by J.Andreina )

(cherry picked from commit 5be5129d93481e9caae6a05b2361c463211a6d55)

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

@@ -459,6 +459,9 @@ Release 2.7.0 - UNRELEASED
 
     HDFS-7054. Make DFSOutputStream tracing more fine-grained (cmccabe)
 
+    HDFS.7849. Update documentation for enabling a new feature in rolling
+    upgrade ( J.Andreina via vinayakumarb )
+
   OPTIMIZATIONS
 
     HDFS-7454. Reduce memory footprint for AclEntries in NameNode.

hadoop-hdfs-project/hadoop-hdfs/src/site/xdoc/HdfsRollingUpgrade.xml

@@ -47,6 +47,15 @@
     These two capabilities make it feasible to upgrade HDFS without incurring HDFS downtime.
     In order to upgrade a HDFS cluster without downtime, the cluster must be setup with HA.
   </p>
+  <p>
+    If there is any new feature which is enabled in new software release, may not work with old software release after upgrade.
+    In such cases upgrade should be done by following steps.
+  </p>
+  <ol>
+    <li>Disable new feature.</li>
+    <li>Upgrade the cluster.</li>
+    <li>Enable the new feature.</li>
+  </ol>
   <p>
     Note that rolling upgrade is supported only from Hadoop-2.4.0 onwards.
   </p>