DVDs: Much needed "shot in the arm" for video servers

The release of DVD-ROMs (Digital Versatile Disk-Read Only Memory), capable of storing an entire 133 minute MPEG-2 movie, coupled with the gradual deployment of high speed networks will give a much needed impetus to Video-on-Demand systems, especially movie-on-demand services. Various memory components such as DVD-ROMs, magnetic RAID towers, and RAM are available, each with different storage and bandwidth capabilities, and each with different costs. To design a video server, we need to optimize the cost factor, yet meet the massive storage, high bandwidth and continuity requirements of video stream delivery.\ud \ud In the first part of this paper we compare DVD-ROM and RAID systems on the basis of two factors-a storage factor and a bandwidth factor. We work out relations to calculate capacity requirements and costs, of these systems given the demands of video delivery. We then propose an architecture for a video server where we deploy three layers of memory functioning as video pumps: DVD-ROM towers, RAID towers, and RAM. We then work out expressions to determine the migration strategy of a movie between these layers of storage so as to optimize on the cost of storage while satisfying the performance requirements. We then show the method by which the amount of storage required for each type of memory in the three layers can be fixed, knowing the usage pattern.\ud \ud In the second part of the paper, we discuss the implementation of a video pump where there is a need to integrate scheduling, admission control, VER stream management, and handling of VCR-like requests such as fast-forward, fast-reverse and pause. We first work out general relations for calculation of the service time of a request knowing the seek overheads and playback rates. We then show that, surprisingly, the C-SCAN algorithm, which incurs the least amount of seek overhead and therefore has a lesser cycle time, is twice as efficient as SCAN in terms of buffer space requirements. We then propose a 'full-load' admission control and scheduling algorithm that operates on a constant cycle time basis. We test the validity of this scheme though a the simulator which we have built based on our disk model and use it to extract parameters such as disk bandwidth utilization factor and buffer sizes needed for a VER load scenario