Coping with Excessive Memory Requirement Under I/O Bandwidth Congestion

Abstract

In this paper, we investigate the buffer requirement in retrieving the continuous media streams from the disk subsystem. Memory buffer is used to synchronize asynchronous disk read operation and synchronous playback operation. In supporting a set of continuous media playbacks, as aggregate bandwidth required increases, larger amount of buffer needs to ibe allocated. This characteristics originates from the increase in cycle length. It is well known fact that as disk utilization approaches I 00%, total buffer to support the playbacks increases extremely fast. Conservative estimation on disk usage is thus advised in designing disk subsystem for continuous media server. However, in the practical situation aggregate bandwidth may increase over the expected value, and may consume excessive amount of buffer memory. The focus of this is to an algorithm coping with this excessive buffer requirement under bandwidth congestion. We argue that in a large scale continuous media server, where user access pattern is biased and there are frequent request arrivals, it is not necessary to maintain the playback directly from the disk for each request in supporting a set of timely interleaved playbacks. We define two mechanisms to service a playback request, namely disk mode and memory mode. In memory mode, request is supplied data blocks which was loaded by preceding request. We develop an efficient algorithm to determine the optimal service mode for a set of playback request minimizing overall buffer requirement.