This article provides a retrospective of our original paper by the same title in the Proceedings of the First ACM Conference on Multimedia, published in 1993. This article examines the problem of disk scheduling in a multimedia I/O system. In a multimedia server, the disk requests may have constant data rate requirements and need guaranteed service. We propose a new scheduling algorithm, SCAN-EDF, that combines the features of SCAN type of seek optimizing algorithm with an Earliest Deadline First (EDF) type of real-time scheduling algorithm. We compare SCAN-EDF with other scheduling strategies and show that SCAN-EDF combines the best features of both SCAN and EDF. We also investigate the impact of buffer space on the maximum number of video streams that can be supported.We show that by making the deadlines larger than the request periods, a larger number of streams can be supported.We also describe how we extended the SCAN-EDF algorithm in the PRISM multimedia architecture. PRISM is an integrated multimedia server, designed to satisfy the QOS requirements of multiple classes of requests. Our experience in implementing the extended SCAN-EDF algorithm in a generic operating system is discussed and performance metrics and results are presented to illustrate how the SCAN-EDF extensions and implementation strategies have succeeded in meeting the QOS requirements of different classes of requests.

In this paper, we describe our experience in building an integrated multimedia storage system, Prism. Our current Linux-based implementation of Prism provides three levels of service: deadline guarantees for periodic applications, best-effort better response times for interactive applications and starvation-free throughput guarantees for aperiodic applications. Prism separates resource allocation from resource scheduling. Resource allocation is controlled across the service classes by a system-wide policy and service class specific admission controllers. Resource scheduling is done at the resources. This separation allows Prism to be deployed even when the storage system is separated on a network from the file system.We report on the important aspects of Prism architecture, innovations required to build Prism on top of Linux and lessons learned during the implementation and testing of Prism. We present experimental results to show that Prism achieves its goals in supporting multiple service classes within a single system. We compare Prism against standard Linux operating system to show the impact of our approach.