HDFS-14223. RBF: Add configuration documents for using multiple sub-clusters. Contributed by Takanobu Asanuma.

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/resources/hdfs-rbf-default.xml

@@ -275,7 +275,8 @@
     <name>dfs.federation.router.file.resolver.client.class</name>
     <value>org.apache.hadoop.hdfs.server.federation.resolver.MountTableResolver</value>
     <description>
-      Class to resolve files to subclusters.
+      Class to resolve files to subclusters. To enable multiple subclusters for a mount point,
+      set to org.apache.hadoop.hdfs.server.federation.resolver.MultipleDestinationMountTableResolver.
     </description>
   </property>
 

hadoop-hdfs-project/hadoop-hdfs-rbf/src/site/markdown/HDFSRouterFederation.md

@@ -404,7 +404,7 @@ Forwarding client requests to the right subcluster.
 
 | Property | Default | Description|
 |:---- |:---- |:---- |
-| dfs.federation.router.file.resolver.client.class | `org.apache.hadoop.hdfs.server.federation.resolver.MountTableResolver` | Class to resolve files to subclusters. |
+| dfs.federation.router.file.resolver.client.class | `org.apache.hadoop.hdfs.server.federation.resolver.MountTableResolver` | Class to resolve files to subclusters. To enable multiple subclusters for a mount point, set to org.apache.hadoop.hdfs.server.federation.resolver.MultipleDestinationMountTableResolver. |
 | dfs.federation.router.namenode.resolver.client.class | `org.apache.hadoop.hdfs.server.federation.resolver.MembershipNamenodeResolver` | Class to resolve the namenode for a subcluster. |
 
 ### Namenode monitoring