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In this paper we compare techniques for storage and real-time retrieval of Variable Bit Rate (VBR) video data for multiple simultaneous users. The motivation for considering VBR is that video results in inherently time varying data, and as such, with the same average bit rate, higher quality can be achieved with VBR than with Constant Bit Rate (CBR). We propose and compare the following three classes of VBR data placement and retrieval techniques: Constant Time Length (CTL) places and retrieves data in blocks corresponding to equal playback durations, Constant Data Length (CDL) places and retrieves constant-sized data blocks, and a hybrid solution uses CDL placement but retrieves a variable number of blocks in each service round. We have found that CTL data placement has much lower buffer requirements than CDL but suffers from fragmentation during video editing. We show hybrid placement to have both advantages of high efficiency and low fragmentation. We also address the issue of admission control policies by comparing statistical and deterministic techniques. `Statistical' admission control uses statistics of the stored data to ensure that the probability of `overload' does not exceed a prespecified threshold. `Deterministic' control uses the actual stored video bit traces to regulate the number of admitted users. We consider two types of deterministic admission control: data-limit and ideal deterministic. Data-limit admission control admits users based on precomputing the total amount of data requested by all users in future service rounds. In contrast, ideal deterministic admission control not only precomputes the total amount of data requested, but also assumes we have control of data placement at the disk sector level in order to precompute the future seek and rotation times. We provide a cost/benefit analysis of the above placement/retrieval/admission control techniques and conclude that CTL and hybrid placement/retrieval techniques can reduce the total system cost by up to a factor of 3 in comparison with the strategy of padding the VBR video trace to achieve a constant data rate. 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. We find that all forms of deterministic admission control can outperform statistical, but the greatest gain comes from using ideal deterministic admission control. We note, however, that this admission control may be difficult to implement on standard disk controllers. Finally, we have implemented a full disk model simulator that operates 1000 times faster than the real-time disk. Results using the simulator are very close to those measured on the real disk, making the simulator useful for future experiments.
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