public void write(int offset, ByteBuffer src) throws IOException {
      SeekableByteChannel channel = channelAndPosition.getChannel();
      long position = channelAndPosition.getPosition();
      channel.position(position + offset);
      channel.write(src);
    }
Enables writing and rewriting portions of the file backing an {@link HFileBlock}.
hbase-server/src/test/java/org/apache/hadoop/hbase/io/hfile/TestHFileBlockHeaderCorruption.java