MULTIMEDIA STORAGE SERVERS - A TUTORIAL

On-line access to multimedia information--like books, periodicals, images, video clips, and scientific data--is both possible and cost-effective, thanks to recent advances in computing and communication. Some media, such as audio and video, are classified as continuous because audio samples and video frames, for example, have meaning only when presented in time. The design of multimedia servers thus fundamentally differs from conventional servers as a result of (1) real-time storage and retrieval requirements, as well as (2) large storage space and data transfer-rate requirements of digital multimedia. In this tutorial, the authors highlight the issues involved in meeting these requirements. For example, the critical components in the design of multimedia services are storage servers that support continuous media storage and retrieval, and network subsystems that synchronously deliver media information, on time, to the client sites. In their survey of design issues, the authors present disk-scheduling algorithms (optimized for retrieving multimedia information) for real-time recording and playback. The authors also discuss admission control algorithms that let a multimedia server determine whether new services can be added without the server's violating the real-time requirements of existing ones. In terms of service, the authors assume that performance requirements of multimedia storage servers include meeting all real-time deadlines, although some applications can tolerate missing one occasionally. Servers have several quality-of-service (QoS) categories: (1) Deterministic--All deadlines are guaranteed to be met; (2) Statistical--Deadlines are guaranteed to be met with a certain probability (perhaps 90 percent); and (3) Background--No guarantees are given for meeting deadlines (the server schedules access only when there is time left over after all other service). The authors also examine techniques for efficiently placing media information on individual disks, large disk arrays, and storage device hierarchies. Finally, they describe data structures that a multimedia file system must maintain to facilitate random access and efficient editing.