Minimizing Blocking Probability in a Hierarchical Storage Based VOD Server

I n this paper we investigate the relationship between the blocking probability and the configuration of storage hierarchy in a V O D server. Due t o limited sys tem resources available in a V O D server, some user requests can not be serviced immediately and are thus blocked. I n 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 f r o m the secondary storage m a y increase the blocking probability, and hence degrade the server performance. W e provide a n analytical model o f the blocking probability, which i s to be used t o find the optimal data rate to minimize the blocking probability f o r a given sys tem configuration. B y enforcing the files t o reside a t the staging disk f o r a certain amount of t ime , it is possible to further improve the blocking probability. W e examine the problem of space management for video files o n the staging disks, and devise a policy called Enforced Duration to arbitrates the duration of the file o n the staging disk. Results of a simulation based performance evaluation are presented.