code: @Override
  	protected synchronized void read(long offset, byte[] b) throws IOException {
  		if (byteBuffer != null) {
  			byteBuffer.position((int) offset);
  			byteBuffer.get(b);
  		} else {
  			throw new IOException("Read failed, file " + getPath() + " not mapped for I/O");
  		}
  	}
docs: Reads a number of bytes from the RRD file on the disk
  
  @param offset Starting file offset
  @param b	  Buffer which receives bytes read from the file.