On Thu, Mar 10, 2011 at 12:48 AM, Adarsh Sharma <[email protected]> wrote: > Thanks Harsh, i.e why if we again format namenode after loading some data > INCOMATIBLE NAMESPACE ID's error occurs. > > > Best Regards, > > Adarsh Sharma > > > > > Harsh J wrote: >> >> 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.). >> >> > >
If you do not tell where you NN to store data it stores it to /tmp. And your operating system cleans up temp. The reason for the error you see is datanodes don't like to suddenly connect to new namenodes. So as a safety they do not start up until they are cleared.
