Formatting the NameNode initializes the FSNameSystem in the dfs.name.dir directories, to prepare for use.
The format command typically writes a VERSION file that specifies what the NamespaceID for this FS instance is, what was its ctime, and what is the version (of the file's layout) in use. This is helpful in making every NameNode instance unique, among other things. DataNode blocks carry the namespace-id information that lets them relate blocks to a NameNode (and thereby validate, etc.). -- Harsh J www.harshj.com
