code: public WriteBuffer set(final int index) {
  		if (useMmap) {
  			final ByteBuffer buf = getMmapForIndex(index);
  			if (buf != null) {
  				return new WriteBuffer(this, index, useMmap, buf);
  			}
  		}
  		return new WriteBuffer(this, index, false, bufstack.pop());
  	}
docs: Alloc a WriteBuffer
  
  @param index of block
  @return WriteBuffer