Minimizing blocking probability in a hierarchical storage based VOD server

We investigate the relationship between the blocking probability and the configuration of storage hierarchy in a VOD server. Due to limited system resources available in a VOD server, some user requests can not be serviced immediately and are thus blocked. In general, one would expect the increasing the I/O rate will reduce the request blockage. However, we report the interesting observation that increasing the data rate from the secondary storage may increase the blocking probability, and hence degrade the server performance. We provide an analytical model of the blocking probability, which is to be used to find the optimal data rate to minimize the blocking probability for a given system configuration. By enforcing the files to reside at the staging disk for a certain amount of time, it is possible to further improve the blocking probability. We examine the problem of space management for video files on the staging disks, and devise a policy called enforced duration to arbitrate the duration of the file on the staging disk. Results of a simulation based performance evaluation are presented.