code: boolean addNode(Block b, DatanodeDescriptor node, int replication) {
      // insert into the map if not there yet
      BlockInfo info = checkBlockInfo(b, replication);
      // add block to the data-node list and the node to the block info
      return node.addBlock(info);
    }
docs: returns true if the node does not already exists and is added.
  false if the node already exists.