A model of shared DASD and multipathing
Communications of the ACM
Modeling I/O systems with dynamic path selection, and general transmission networks
SIGMETRICS '82 Proceedings of the 1982 ACM SIGMETRICS conference on Measurement and modeling of computer systems
Theory, Volume 1, Queueing Systems
Theory, Volume 1, Queueing Systems
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The recently introduced Extended Architecture (XA) of large IBM computer systems includes, in the disk I/O area, the ability for an access to be resumed and completed on a path different from the one on which it has been initiated. The expected disk performance improvement due to this feature - known as Dynamic Path Reconnection - is investigated in this note. Two popular double pathing connection schemes are considered: switched substrings (as in IBM 3380 Dynamic Path Selection) and dual ported devices (as in Amdahl 6280 Dynamic Performance Pathing). A simple classical queueing system is used to model missed revolutions when transfer paths are found busy. The numerical results obtained indicate that, depending on load, a substantial reduction in the average I/O time can be expected with the Dynamic Path Reconnection feature. This reduction can top 30% under moderately heavy loads. The accuracy of the analytical model for missed reconnections has been checked using discrete-event simulation.