HDFS federation support multiple namenodes horizontally scales the file system namespace. As the amount of data grows, using a single group of namenodes to manage a single directory, namenode still achieves performance bottlenecks. In order to reduce the pressure of namenode, we can split out the secondary directory, and manager it by a new namenode. This is transparent for users.
For example, nn1 only manager the /user directory, when nn1 achieve performance bottlenecks, we can split out /user/hive directory, and ues nn2 to manager it.
That means core-site.xml should support as follows configuration.