I/O ISSUES IN A MULTIMEDIA SYSTEM

In a multimedia server, disk requests can require constant data rates and guaranteed service. The authors discuss the impact of the real-time nature of I/O requests on various I/O system components as well as the impact of disk scheduling algorithms on the performance of a multimedia system. This article describes a hybrid scheduling algorithm, Scan-EDF (earliest deadline first), which combines a real-time policy such as EDF with a seek-optimizing policy such as CScan (circular Scan). It then shows how Scan EDF can support a larger number of realtime streams and simultaneously provide better response times to aperiodic requests. The authors also investigate the impact of buffer space on the maximum number of video streams that can be supported. Then they show that even more streams can be supported by using delayed deadlines and larger requests. Of the two techniques, they prefer delayed deadlines, which provide better response times to aperiodic requests. When multiple disks are connected to the system through a single bus such as SCSI (for Small Computer Systems Interface), SCSI bus scheduling can add extra delays to individual requests. The authors examine the impact of priority-driven arbitration of a SCSI bus on disk throughput. They then show that deadline extension helps to increase system throughput when multiple disks are connected on a single SCSI bus.