Abstract: A proper parallel I/O service model plays an important role in making an efficient use of the high I/O bandwidth provided by an I/O subsystem. It is pointed out in this paper, that the original DDIO makes little improvement of performance in the small size and fine grained access mode, since the high communication overhead causes a new bottleneck. Then, a new I/O service model IDDIO is proposed, which fits small size and fine grained as well as large size access. IDDIO combines the packing and pipelining technology to reduce the communication overhead. The simulation shows that the IDDIO provides a higher bandwidth than the DDIO.