Cost Analyses for VBR Video Servers

• Authors: Ed Chang;Avideh Zakhor
• Venue: IEEE MultiMedia
• Year: 1996

This article compares techniques for storage and real-time retrieval of variable bit rate (VBR) video data for multiple simultaneous users. The authors compare the following three classes of VBR data placement and retrieval techniques: Constant Time Length (CTL) places and retrieves data in units corresponding to equal playback durations, Constant Data Length (CDL) places and retrieves constant-sized data units, and a hybrid solution uses CDL placement but retrieves a variable number of units per user in each service round. A cost analysis of the above techniques yields the conclusion that CTL and hybrid techniques can reduce the total system cost by up to a factor of 3 in comparison with padding the VBR video trace. For read-only systems, CTL has the lowest cost per user. For writable systems, the hybrid technique achieves a good compromise between low cost and low fragmentation. The authors also address the issue of admission control policies by comparing three techniques. Statistical admission control uses statistics of the stored data to ensure that the probability of server overload does not exceed a prespecified threshold. Data-limit admission control admits users based on precomputing the total amount of data requested by all users in future service rounds. Ideal deterministic admission control assumes we have control of data placement at the disk sector level to precompute all disk access times. Data-limit admission control offers a moderate gain for a moderate increase in implementation complexity, as shown. The authors have implemented a highly accurate full disk model simulator that operates 1,000 times faster than the real-time disk, making it useful for experiments. Finally, they address multiple disk issues and consider applying their data placement and admission control strategies to an interleaved disk array.Readers may contact Chang at Digital Equipment corp., Cambridge Research Lab. One Kendall square, Bldg. 700, Cambride, MA 02139, e-mail [e-mail].