Αναλυση Επιδοσης Συγχρονων Μαγνητικων Δισκων

Abstract

The evolution of computer systems technology is characterized by drastic changes and innovations. The main memory and processors evolved and progressed in a very fast speed, while the secondary memory (the third major factor in computer technology) proceeded in slower rates. Magnetic disks remained the main and dominant medium of permanent and non-volatile data storage, for the last three decades, despite various other technology proposals. Magnetic disks provide long lasting file storage and an additional level in memory hierarchy which is used as a virtual memory during the programs' execution. The present dissertation focuses in studying the performance of various modern magnetic disk drives. It examines various disk topologies and it includes specific surveys for the disks with two read/write heads per disk surface. The performance is measured by different metrics such as minimum, average and maximum seek times, number of disk cylinders accessed, number of requested clusters etc. χ ABSTRACT Comparisons axe made and the seek time is proven to be a very important factor needed in the overall performance measurement.Chapter 3 refers to various disk models for disk systems with two read/write heads per surface, where the heads remain always in fixed distance. There is a further examination of the Traveling Salesman Problem which is adapted to the two headed disk model. New formulae are derived for the servicing cost of various query types refering to a range of consecutive cylinder positions. In the case of partial match queries, both the expected number of the heads' stops and the expected number of the disk cylinder clusters are calculated for the two fixed headed disk model. Comparisons with the conventional one headed disk are made and the new results are discussed in detail.Chapter 4 considers the theoretical model of disks with two heads moving autonomously. Each head can sweep all cylinders of the disk surface and the requests are served by the nearerserver rule. The expected seek time is calculated analytically. There is also another servicing approach based on a greedy algorithm, in order to minimize the expected seek time. The greedy approach exploits the two headed scheme by placing one head on an appropriate position while the other head satisfies the request. Certain formulae for the expected seek time of both servicing approaches is derived.Chapter 5 presents in detail the modern systems of parallel disks, which comprise of either two disk drives (mirrored disks) or more than two drives (shadowed disks). In the present dissertation there is an analytic study of parallel disk systems where each disk has two heads either in fixed distance or in autonomous movement. A new mathematical approach for the calculation of seek distances traveled by the heads, is presented by using Markov chain tech niques. The study deals with the general case of shadowed disks by distinguishing seeks in dependent and independent ones. The dependency of seeks results in two different metrics for the expected seek distance. T...