HDFS scales by adding DataNodes in the cluster. HDFS allows DataNodes to be added to a running cluster and offers tools to manually rebalance the data blocks when cluster nodes are added, which can be done without shutting the file system down. Since NameNode stores all metadata in memory (for faster operations), HDFS is limited by how much memory the NameNode has. This limitation is handled through HDFS Federation, which was introduced in the 2.x release. HDFS Federation allows a cluster to scale by adding NameNodes, each of which manages a portion of the filesystem namespace.