Division-of-labor between server and P2P for streaming VoD

We consider a P2P-assisted content storage and delivery system to support a streaming Video-on-Demand (VoD) service. In this system, the peers are part of the service provider (e.g. set-top boxes) with limited storage space. Servers with ample storage and bandwidth are deployed to guarantee the availability and quality, but it is desirable to minimize the server utilization to reduce costs. Based on experience of implementing a deployed P2P VoD system, it was suggested in [1] that a movie´s availability should be proportional to the movie´s popularity. Based on further refinement, it is observed [2] that performance can be further improved by more (than proportional) availability for cold movies in P2P system. In this paper, we show that as the number of movies becomes large and there is some skewness in movie popularity, then one cannot expect the P2P part of the system to reduce server load as well as provide availability to all movies at the same time. It is a trade-off between coverage of movies and streaming throughput provided by the P2P system. If the goal is to minimize server load, under some reasonable conditions, we show that it is best to store and replicate only the hottest K* movies in the P2P part of the system. We also study the relationship between the skewness of the movie popularity distribution, P2P resources and the value of K*. Finally, we use simulation to validate our results.