What does a secondary namenode do in Hadoop

Question

Hey. When I start all the daemons, I also see that there is a daemon called "Secondary Namenode". I wanted to know what is it used for because I cant find any operations to do on it. And does it start automatically?

Answer 1

The Secondary namenode is mainly used as a backup of namenode. The Secondary namenode collects checkpoints of metadata in namenode and then use it in case of namenode failure. 

We need not use Secondary namenode because standby namenode keep its state synchronized with the Active namenode.

When you use the ./start-all.sh command, it starts the Secondary namenode but you can choose not to start it by starting other daemons individually.

Comments

you are confusing 2nddary nn with backup node!

The Secondary NameNode is one which constantly reads all the file systems and metadata from the RAM of the NameNode and writes it into the hard disk or the file system. It is responsible for combining the EditLogs with FsImage from the NameNode.

Secondary NameNode downloads the EditLogs from the NameNode at regular intervals and applies to FsImage. The new FsImage is copied back to the NameNode, which is used whenever the NameNode is started the next time.