Cloudera Docs

Enabling HDFS HA

An HDFS high availability (HA) cluster uses two NameNodes -- an active NameNode and a standby NameNode. Only one NameNode can be active at any point in time.

HDFS HA depends on maintaining a log of all namespace modifications in a location available to both NameNodes, so that in the event of a failure, the standby NameNode has up-to-date information about the edits and location of blocks in the cluster.