Merge HDFS-3091. Update the usage limitations of ReplaceDatanodeOnFailure policy in the config description for the smaller clusters. Contributed by Nicholas.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1302633 13f79535-47bb-0310-9956-ffa450edef68

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

@@ -145,6 +145,9 @@ Release 0.23.3 - UNRELEASED
 
     HDFS-3111. Missing license headers in trunk. (umamahesh)
 
+    HDFS-3091. Update the usage limitations of ReplaceDatanodeOnFailure policy in
+    the config description for the smaller clusters. (szetszwo via umamahesh)
+
   OPTIMIZATIONS
     HDFS-2477. Optimize computing the diff between a block report and the
                namenode state. (Tomasz Nykiel via hairong)

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

@@ -371,6 +371,12 @@
 
     This is a site-wide property to enable/disable the feature.
 
+    When the cluster size is extremely small, e.g. 3 nodes or less, cluster
+    administrators may want to set the policy to NEVER in the default
+    configuration file or disable this feature.  Otherwise, users may
+    experience an unusually high rate of pipeline failures since it is
+    impossible to find new datanodes for replacement.
+
     See also dfs.client.block.write.replace-datanode-on-failure.policy
   </description>
 </property>