How does HDFS Federation help HDFS Scale horizontally?

A.
HDFS Federation improves the resiliency of HDFS in the face of network issues by removing
the NameNode as a single-point-of-failure.

B.
HDFS Federation allows the Standby NameNode to automatically resume the services of an
active NameNode.

C.
HDFS Federation provides cross-data center (non-local) support for HDFS, allowing a cluster
administrator to split the Block Storage outside the local cluster.

D.
HDFS Federation reduces the load on any single NameNode by using the multiple,
independent NameNode to manage individual pars of the filesystem namespace.

Explanation:
HDFS FederationIn order to scale the name service horizontally, federation uses
multiple independent Namenodes/Namespaces. The Namenodes are federated, that is, the
Namenodes are independent and don’t require coordination with each other. The datanodes are
used as common storage for blocks by all the Namenodes. Each datanode registers with all the
Namenodes in the cluster. Datanodes send periodic heartbeats and block reports and handles
commands from the Namenodes.
Apache Hadoop 2.0.2-alpha
http://hadoop.apache.org/docs/current/