What does Secondary NameNode ? And what is the use ?

Secondary NameNode :


- This Secondary NN is not a hot backup of the NameNode. It cannot be used to the event of a NameNode failure.

- This Daemon, periodically synchronizes with the NameNode block index. During the synchronizing process, the Secondary NN retrieves the current NameNode image and edit logs, merges them together, and then sends the merged image back to the NameNode.