code: public BlocksWithLocations getBlocks(DatanodeInfo datanode, long size)
    throws IOException {
      if(size <= 0) {
        throw new IllegalArgumentException(
          "Unexpected not positive size: "+size);
      }
  
      return namesystem.getBlocks(datanode, size);
    }
docs: return a list of blocks & their locations on <code>datanode</code> whose
  total size is <code>size</code>
  
  @param datanode on which blocks are located
  @param size total size of blocks