XXX (ab) Bulk of this method is copied verbatim from {@link DFSClient}, which is
   * bad. Both places should be refactored to provide a method to copy blocks
   * around.
XXX (ab) Bulk of this method is copied verbatim from {@link DFSClient}, which is
   * bad. Both places should be refactored to provide a method to copy blocks
   * around. | NamenodeFsck.java