Using tertiary storage in video-on-demand servers

Video-on-demand is a new entertainment service that will soon be widely available. A small amount of material is very popular, while large amounts of material are viewed less frequently. This skew can be exploited by using a storage hierarchy, storing the less frequently viewed videos in lower-cost tertiary storage. This paper studies the use of tertiary storage for videos. Tertiary storage devices such as optical disks and magnetic tapes can be used to a) deliver data directly to viewers, or b) to stage data to disk for viewing. Analysis of these modes yields guidelines for server design. Examining device characteristics, workload characteristics, and cost, the two modes are compared to each other and to playing from disk. The data placement decision depends on the fraction of time a stream of a video is active. At current costs, videos having an active stream less than a third of the time should reside on tertiary storage. When a tertiary library has a much higher data rate than the video rate, videos should be staged disk for playing. Otherwise, they should be played directly from tertiary store.