code: DatanodeBlockInfo get(int namespaceId, Block block) {
      checkBlock(block);
      NamespaceMap nm = getNamespaceMap(namespaceId);
      if (nm == null) {
        return null;
      }
      return nm.getBlockInfo(block);
    }
docs: Get the meta information of the replica that matches both block id and
  generation stamp
  
  @param namespaceId
  @param block
  block with its id as the key
  @return the replica's meta information
  @throws IllegalArgumentException
  if the input block or block pool is null