Your Hadoop cluster contains nodes in three racks.

The NameNode will update the dfs.hosts property to include machines running the DataNode daemon on the next NameNode reboot or with the command dfsadmin –refreshNodes

No new nodes can be added to the cluster until you specify them in the dfs.hosts file

Any machine running the DataNode daemon can immediately join the cluster

Presented with a blank dfs.hosts property, the NameNode will permit DataNodes specified in mapred.hosts to join the cluster