Identify the function performed by the Secondary NameNode daemon on a cluster configured to
run with a single NameNode.

A.
In this configuration, the Secondary NameNode performs a checkpoint operation on the files by
the NameNode.

B.
In this configuration, the Secondary NameNode is standby NameNode, ready to failover and
provide high availability.

C.
In this configuration, the Secondary NameNode performs deal-time backups of the NameNode.

D.
In this configuration, the Secondary NameNode servers as alternate data channel for clients to
reach HDFS, should the NameNode become too busy.

Explanation:
The term “secondary name-node” is somewhat misleading. It is not a name-node in
the sense that data-nodes cannot connect to the secondary name-node, and in no event it can
replace the primary name-node in case of its failure.
The only purpose of the secondary name-node is to perform periodic checkpoints. The secondary
name-node periodically downloads current name-node image and edits log files, joins them into
new image and uploads the new image back to the (primary and the only) name-node.
So if the name-node fails and you can restart it on the same physical node then there is no need
to shutdown data-nodes, just the name-node need to be restarted. If you cannot use the old node
anymore you will need to copy the latest image somewhere else. The latest image can be found
either on the node that used to be the primary before failure if available; or on the secondary
name-node. The latter will be the latest checkpoint without subsequent edits logs, that is the most
recent name space modifications may be missing there. You will also need to restart the whole
cluster in this case.
Hadoop Wiki, What is the purpose of the secondary name-node?