code: DatanodeBlockInfo add(int namespaceId, Block block,
        DatanodeBlockInfo replicaInfo) {
      checkBlock(block);
      NamespaceMap nm = getNamespaceMap(namespaceId);
      if (nm == null) {
        return null;
      }
      replicaInfo.setBlock(block);
      if (datasetDelta != null) {
        datasetDelta.addBlock(namespaceId, block);
      }
      return nm.addBlockInfo(block, replicaInfo);
    }
docs: Add a replica's meta information into the map
  
  @param namespaceId
  @param replicaInfo
  a replica's meta information
  @return previous meta information of the replica
  @throws IllegalArgumentException
  if the input parameter is null