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A multimedia server requires a real-time disk-scheduling algorithm to deliver isochronous data for real-time streams. Traditional disk-scheduling algorithms focus on providing good quality in a best effort manner. In this paper, we propose a novel real-time disk-scheduling algorithm called WRR-SCAN (weighted-round-robin-SCAN) to provide quality guarantees for all in-service streams encoded at variable bit rates and bounded response times for aperiodic jobs. WRR-SCAN divides a real-time stream into guaranteed jobs and optional jobs. The admission control admits a stream as long as its guaranteed jobs are satisfied. Such a decision is made in 0(1) time as WRR-SCAN reserves a fixed weight for each stream. WRR-SCAN incorporates an aggressive policy to dynamically reclaim unused bandwidth during runtime. The reclaimed bandwidth is used to serve optional jobs or more aperiodic jobs. We conducted a set of simulations to compare WRR-SCAN with a set of referred disk-scheduling algorithms. The evaluations are conducted on a commonly used disk simulator with traces from a real multimedia server. The experimental results show that WRR-SCAN provides significantly better quality for real-time streams and yields considerably shorter response times for aperiodic jobs.