Recent technological advances in digital signal processing, data compression techniques, and high speed communication networks have made Video-on-Demand (VOD) servers feasible. A challenging task in such systems is servicing multiple clients simultaneously while satisfying real-time requirements of continuous delivery of objects at specified rates. To accomplish these tasks and realize economies of scale associated with servicing a large user population, a VOD server requires a large disk subsystem. Although a single disk is fairly reliable, a large disk farm can have an unacceptably high probability of disk failure. Furthermore, due to real-time constraints, the reliability and availability requirements of VOD systems are even more stringent than those of traditional information systems. In this paper we discuss some of the main issues and tradeoffs associated with providing fault tolerance in multidisk VOD systems.
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