HDFS-7934. Update RollingUpgrade rollback documentation: should use bootstrapstandby for standby NN. Contributed by J. Andreina.

(cherry picked from commit b172d03595d1591e7f542791224607d8c5fce3e2)
(cherry picked from commit 725621d95a833f813e2164e32b33984e1c1ee29a)

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

@@ -23,6 +23,9 @@ Release 2.7.1 - UNRELEASED
     HDFS-8151. Always use snapshot path as source when invalid snapshot names
     are used for diff based distcp. (jing9)
 
+    HDFS-7934. Update RollingUpgrade rollback documentation: should use
+    bootstrapstandby for standby NN. (J. Andreina via jing9)
+
 Release 2.7.0 - UNRELEASED
 
   INCOMPATIBLE CHANGES

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

@@ -263,14 +263,16 @@
   <p>
     Rollback from a newer release to the pre-upgrade release is always supported.
     However, it cannot be done in a rolling fashion.  It requires cluster downtime.
+    Suppose <em>NN1</em> and <em>NN2</em> are respectively in active and standby states.
     Below are the steps for rollback:
   </p>
   <ul>
     <li>Rollback HDFS<ol>
       <li>Shutdown all <em>NNs</em> and <em>DNs</em>.</li>
       <li>Restore the pre-upgrade release in all machines.</li>
-      <li>Start <em>NNs</em> with the
+      <li>Start <em>NN1</em> as Active with the
         "<a href="#namenode_-rollingUpgrade"><code>-rollingUpgrade rollback</code></a>" option.</li>
+      <li>Run `-bootstrapStandby' on NN2 and start it normally as standby.</li>
       <li>Start <em>DNs</em> with the "<code>-rollback</code>" option.</li>
     </ol></li>
   </ul>