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As the size of cluster becomes larger, the process ability of a cluster increases rapidly. Users will exploit this increased power to run scientific, physical and multimedia applications. These kinds of data-intensive applications require high performance storage subsystem. Parallel storage system such as RAID is widely used in today's clusters. In this paper, we bring out a "greedy" I/O scheduling method that utilizes Scatter and Gather operations inside the PCI-SCSI adapter to combine as many I/O operations within the same disk as possible. In this way we reduce the numbers of I/O operations and improve the performance of the whole storage system. After analyzing RAID control strategy, we find out that I/O commands' combination may also bring up data movement in memory and this kind of movement will increase the system's overhead. The experiment results in our real time operating environment show that a better performance can be achieved. The longer the data length is, the better improvement we can get, in some case, we can even get over 40% enhancement.