code: private synchronized void readBlockSizeInfo() throws IOException {
      if (!transferBlockSize) {
        return;
      }
      blkLenInfoUpdated = true;
      isBlockFinalized = in.readBoolean();
      updatedBlockLength = in.readLong();
      if (dataTransferVersion >= DataTransferProtocol.READ_PROFILING_VERSION) {
        readDataNodeProfilingData();
      }
      
      if (LOG.isDebugEnabled()) {
        LOG.debug("ifBlockComplete? " + isBlockFinalized + " block size: "
            + updatedBlockLength);
      }      
    }
docs: Read the block length information from data stream
  
  @throws IOException