Multimedia Tools and Applications, 19, 279–296, 2003 c  2003 Kluwer Academic Publishers. Manufactured in The Netherlands. A Statistical Admission Control Scheme for Continuous Media Servers Using Caching JIN B. KWON jbkwon@dcslab.snu.ac.kr HEON Y. YEOM yeom@dcslab.snu.ac.kr Department of Computer Science, Sunmoon University, Kalsan-ri 100, Tangeong-Myeon, Asan, Chungnam, 336-708, Korea Abstract. In continuous media servers, disk load can be reduced by using buffer cache. In order to utilize the saved disk bandwidth by caching, a continuous media server must employ an admission control scheme to decide whether a new client can be admitted for service without violating the requirements of clients already being serviced. A scheme providing deterministic QoS guarantees in servers using caching has already been proposed. Since, however, deterministic admission control is based on the worst case assumption, it causes the wastage of the system resources. If we can exactly predict the future available disk bandwidth, both high disk utilization and hiccup-free service are achievable. However, as the caching effect is not analytically determined, it is difficult to predict the disk load without substantial computation overhead. In this paper, we propose a statistical admission control scheme for continuous media servers where caching is used to reduce disk load. This scheme improves disk utilization and allows more streams to be serviced while maintaining near-deterministic service. The scheme, called Shortsighted Prediction Admission Control (SPAC), combines exact prediction through on-line simulation and statistical estimation using a probabilistic model of future disk load in order to reduce computation overhead. It thereby exploits the variation in disk load induced by VBR-encoded objects and the decrease in client load by caching. Through trace-driven simulations, it is demonstrated that the scheme provides near-deterministic QoS and keeps disk utilization high. Keywords: continuous media servers, admission control, QoS, caching, storage systems 1. Introduction Recent advances in computing and communication technologies have made it feasible as well as economically viable to provide on-line access to continuous media objects such as video or audio over high speed networks. The fundamental problem in developing continu- ous media servers is that audio and video differ from image and text in their characteristics, and hence require different techniques for their organization and management. Continuous Media (CM) objects such as audio and video objects which consist of sequences of media quanta(audio samples or video frames), convey meaning only when presented continu- ously in time. In order to ensure continuous delivery of video or audio, various resources, such as disk bandwidth, memory, network bandwidth and so on, should be reserved before commencing playback of a stream. Read-ahead buffering, where blocks are read and buffered ahead of the time they are delivered to the network, has been introduced [5]. Since CM objects usually require high disk bandwidth, it is important to reduce disk I/O in continuous media servers to service the CM objects. Storage systems can reduce the number of disk I/O operations by sharing