@Override
  public HFileBlock retain() {
    bufWithoutChecksum.retain();
    return this;
  }
@return the on-disk size of the next block (including the header size and any checksums if
        present) read by peeking into the next block's header; use as a hint when doing a read
        of the next block when scanning or running over a file.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java