Hi all, I am trying to get a good understanding of how Hadoop works, for my undergraduate project. I have the following questions/doubts :
1. Why does namenode store the blockmap (block to datanode mapping) in the main memory for all the files, even those that are not used? 2. Why cant namenode move out a part of the blockmap from main memory to a secondary storage device, when free space in main memory becomes scarce ( due to large number of files) ? 3. Why cant the blockmap be constructed when a file is requested (by a client) and then be cached for later accesses?
