[Hadoop Wiki] Update of "UnknownHost" by SteveLoughran

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The "UnknownHost" page has been changed by SteveLoughran:
https://wiki.apache.org/hadoop/UnknownHost?action=diff&rev1=4&rev2=5

Comment:
add section on HDFS HA, as it's clearly recurrent

  
  Unless the route cause has been identified, the problem may return.
  
+ 
+ == Unknown Host Exception in HA HDFS ==
+ 
+ This exception surfaces when setting up HA HDFS.
+ 
+ [[https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/HDFSHighAvailabilityWithQJM.html#Configuration_details|As documented], HA HDFS requires you to list the namenode URLs of a cluster in the property {{{edfs.ha.namenodes.mycluster}}}, where "mycluster" is the name of your HA cluster.
+ 
+ {{{
+ <property>
+   <name>dfs.ha.namenodes.mycluster</name>
+   <value>nn1,nn2</value>
+ </property>
+ }}}
+ 
+ Then for the filesystem URL, you use the name of the cluster:
+ 
+ {{{
+ <property>
+   <name>fs.defaultFS</name>
+   <value>hdfs://mycluster</value>
+ </property>
+ }}}
+ 
+ If you get an Unknown Host Exception, and the host is the name of your HA cluster, here {{{mycluster}}}, then it means that the HDFS client hasn't recognized that this is an HA cluster, and instead tried to connect to it directly on the default HDFS port.
+ 
+ '''The dfs.ha.namenodes.mycluster property is unset or the cluster name is inconsistent across the properties'''. Check your config and try again.
+ 
+ 
+ 
  Finally, because this is a configuration problem, filing bug reports is not going to help. They will only be closed as [[http://wiki.apache.org/hadoop/InvalidJiraIssues|Invalid Issues]]