Estimating response time for auxiliary memory configurations with multiple movable-head disk modules
Proceedings of the 1st International Conference on Very Large Data Bases - VLDB '75
Abst ract The hardware architecture for a large data base application often involves the use of movable-head disk modules for auxiliary memory. This paper considers design calculations for such systems and is divided into two main sections: (1) A survey is given of the literature dealing with queueing models for multiple-module movable head disk configurations. Results for these models allow a system designer to estimate the performance of a specified auxiliary memory configuration; for
... ration; for example, the average file response time is one result typically given. References are also provided to papers related to slngl.= movablehead disks which Lescr be techniques for estimating seek time distributiuns, queueing models for channel operations. etc. which ara useful for the multiple-module case. (2) A simple method is provided for estimating the average response time for a multi-module configuration of movable-head disk units attached to a single block multiplexer channel. The technique is a synthesis of a method described by Seaman, Lind, and Wilson for analyzing a similar configuration having a selector channel and variations of a method for treating a block multiplexer channel described by Fuller and Baskett. Seaman~~. find the mean response time by viewing the operation of each disk module as an M/G/l queueing system using the FCFS discipline, where the service time is the sum of the seek time, channel waiting time, and channel service time (rotational delay plus data transfer time). The channel operation in turn is analyzed using the l'machineinterference" model (i .e-., finite-Polsson-source. single-server system with exponential service time distribution). Fuller and Baskett treat the operation of a channel wlt~rotational position sensing by means of a queueing model with Persson arrivals (infinltesource) and service process consIsting of (a) two exponential stages corresponding to rotational delay and data transmission time, respectively, where the first stage has variable service (2) rate which is a function of the number of requests at the channel, or (b) one exponential stage with variable service rate dependent on number of requests at the channel system. The proposed technique involves the use of the method of Seaman~~. but replaces the machine-interference model for channel operation with either of two finite-source queueing models similar to those of Fuller and Baskett. Introduction The architecture of the auxtliary memary subsystem plays a critical role in determining the overall performance of a large data base system. For systems of this type, economic considerations often require that movabJehead disk units be employed because these devices offer lower cost per bit of secondary storage than that for drums and fixed-head disks yet are capable of achieving reasonable performance levels. This paper is concerned with certain design calculations of interest to a systems analyst, and the material covered is very relevant to the problem of hardware architecture for large data bases. The body of this paper is divided into twa main sections as follows: A survey is given of tt.e: 4ueueing models fo" ii'IXi Ilary m'lnlOry subsystems which have previously appeared in the literature. An approximate method is~resented for estimating the mean response time for a multi-module auxiliary memory subsystem~!lth movablehead disks and a single block multiplexer channel which employs rotational position sensing.