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Abstract:

We address the problem of assigning blocks of video streams to disks in News on Demand video servers. Specifically, given the common assumption that video streams are striped across the disks, we consider how to map successive blocks to disks to maximize performance. Our work begins by noting that the commonly proposed round-robin assignment, in which successive blocks of every stream are mapped to successive disks, provides the guarantee that once a video stream starts it will never miss a deadline, but that the latency to start a stream may be high due to the "convoy effect." We propose and analyze an alternative scheme, the random mapping of successive blocks to disks. Using both analytic and simulation techniques, we show that random mapping and replication significantly reduce the latency to start a new stream, and the number of missed deadlines is negligible. Further, we prove that with enough copies, no request sequence will miss any deadlines, for loads that are a significant fraction of the maximum supportable by the hardware. 1

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