Models of DASD Subsystems with Multiple Access Paths: A Throughput-Driven Approach

A throughput-driven approach to the performance analysis of direct access storage devices (DASD´s), i.e., disks, is presented for a model of a block-multiplexor channel system with multiple transfer paths. The approach relies on a step-wise analysis of shared components within the data transfer path: head of string, control unit, channel, referred to as blocking points. In order to derive the service time of an I/O request, each blocking point is analyzed separately, using essentially a classical loss-system model. The advantages of the method are its relative simplicity and applicability to a large number of configurations. Transient and boundary effects, including the miss probability on second and subsequent reconnection attempts, and the device reconnection window (lead time), are also discussed. The inclusion of the approach in a larger system model, as well as its relationship with previous work on DASD analysis is considered. Numerical results illustrate the accuracy of the method.