Minimizing latency and jitter for large-scale multimedia repositories through prefix caching

摘要

Multimedia data poses challenges for efficient storage and retrieval due to its large size and playback timing requirements. For applications that store very large volumes of multimedia data, hierarchical storage offers a scalable and economical alternative to store data on magnetic disks. In a hierarchical storage architecture data is stored on a tape or optical disk based tertiary storage layer with the secondary storage disks serving as a cache or buffer. Due to the need for swapping media on drives, retrieving multimedia data from tertiary storage can potentially result in large delays before playback (startup latency) begins as well as during playback (jitter). In this paper we address the important problem of reducing startup latency and jitter for very large multimedia repositories. We propose that secondary storage should not be used as a cache in the traditional manner - instead, most of the secondary storage should be used to permanently store partial objects. Furthermore, replication is employed at the tertiary storage level to avoid expensive media switching. In particular, we show that by saving the initial segments of documents permanently on secondary storage, and replicating them on tertiary storage, startup latency can be significantly reduced. Since we are effectively reducing the amount of secondary storage available for buffering the data from tertiary storage, an increase in jitter may be expected. However, our results show that the technique also reduces jitter, in contrast to the expected behavior. Our technique exploits the pattern of data access. Advance knowledge of the access pattern is helpful, but not essential. Lack of this information or changes in access patterns are handled through adaptive techniques. Our study addresses both single and multiple-user scenarios. Our results show that startup latency can be reduced by as much as 75% and jitter practically eliminated through the use of these techniques.