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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]]
