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

Namenode contains metadata about the Hadoop filesystem.

This command (hadoop-1.2.1$ bin/hadoop namenode -format) will format whole Hadoop distributed file system(HDFS). So if you run this command on existing filesystem you will lose all your data.