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In this paper we look at the problem of modelling and analysing an I/O subsystem. There are two conventional ways to look at this problem. Tqe first is to model the I/O subsystem as an MIGII queue, thus neglecting the CPU influence on the arrival stream. The second way is to use the central server model, treating the nonexponentially distributed I/O service times .as exponentially distributed.Here we t:J to combine the benefits of both methods. We use a closed network model and analyse this by using an adjusted mean-value scheme. In this scheme service times depend on the number of clients in system. We make use of the residual lifetime distribution of the client in disk service. Some~xperi ments show that the model yields accurate results.

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ABSTRACfDuring the last two decades many interesting and useful results have been obtained in the area of queueing networks. It has been shown that the queueing network model is a powerful tool in computer perfonnance analysis. In this paper we report on some of the difficulties we met in a perfonnance study for the upgrading of the VAX-cluster at the Eindhoven University of Technology. Our conclusion has to be that there are sufficiently many queueing network models and techniques for analyzing them, but that for accurate perfonnance predictions the behaviour of memory contention is not well understood.