Performance of an I/O channel with multiple paging drums

• Authors: Samuel H. Fuller
• Venue: SIGME '73 Proceedings of the 1973 ACM SIGME symposium
• Year: 1973

For rotating storage units, a paging drum organization is known to offer substantially better response time to I/O requests than is a more conventional (file) organization [Abate and Dubner, 1969; Fuller and Baskett, 1972 ]. When several, asynchronous paging drums are attached to a single I/O channel, however, much of the gain in response time due to the paging organization is lost; this article investigates the reasons for this loss in performance. A model of an I/O channel with multiple paging drums is presented and we embed into the model a Markov chain that closely approximates the behavior of the I/O channel. The analysis then leads to the moment generating function of sector queue size and the Laplace-Stieltjes transform of the waiting time. A significant observation is that the expected waiting time for an I/O request to a drum can be divided into two terms: one independent of the load of I/O requests to the drum and another that monotonically increases with increasing load. Moreover, the load varying term of the waiting time is nearly proportional to (2 − 1/k) where k is the number of drums connected to the I/O channel. The validity of the Markov chain approximation is examined in several cases by a comparison of the analytic results to the actual performance of an I/O channel with several paging drums.