What the command “hadoop namenode -format” will do

Question

I am trying to learn Hadoop by following a tutorial and trying to do pseudo-distributed mode on my machine.

My core-site.xml is:

Answer 1

Actually formatting a Namenode will not format the Datanode.

It will just format the contents of your namenode (which contains details of datanode). Your namenode will no longer know where your data is. Also namenode -format will assign a new namespace ID to the namenode

You have to change your namespaceID in your datanode to make your datanode work. This will be at dfs/data/current/VERSION

There is a JIRA open now for the same suggesting to format Datanode as well when you format Namenode. HDFS-107