@Override
  public long heapSize() {
    long size = FIXED_OVERHEAD;
    size += fileContext.heapSize();
    if (bufWithoutChecksum != null) {
      // Deep overhead of the byte buffer. Needs to be aligned separately.
      size += ClassSize.align(bufWithoutChecksum.capacity() + MULTI_BYTE_BUFFER_HEAP_SIZE);
    }
    return ClassSize.align(size);
  }
Returns a byte stream reading the data(excluding header and checksum) of this block
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java