THE FRACTAL STRUCTURE OF DATA REFERENCE- P28:For purposes of understanding its performance, a computer system is traditionally viewed as a processor coupled to one or more disk storage devices, and driven by externally generated requests (typically called transactions). Over the past several decades, very powerful techniques have become available to the performance analyst attempting to understand, at a high level, the operational behavior of such systems. | Disk Applications A Statistical View 125 existed between storage cost and access density. The model is calibrated based upon and reflects the recent history of disk storage use. If storage costs fall then access density should also be expected to fall but at a slower rate. For example a factor-of-two drop in storage costs should be expected to cause a drop in application access densities by approximately a factor of . The concept of usable capacity is helpful in talking about the results of the deployable applications model. The usable capacity is the level of capacity use at which a disk s i o capability is exhausted. For a new disk to be in balance all of its physical capacity must be usable. In general this will require improvements in disk performance to go along with any increases in disk capacity. If all of the capacity is not usable then the effective storage cost will exceed the physical storage cost in proportion to the ratio ofphysical to usable capacity. Our overall objective has been to reconcile two contrasting views one emphasizing storage cost and the other access density. We have found in the deployable applications model some basis for both viewpoints 1. Those concerned with the feasibility ofapplications are justified in focusing on storage cost because by this is the primary driver that determines which applications are cost-effective to deploy. 2. Those concerned with system management are justified in emphasizing access density because the effective cost of a given disk technology depends in part upon its ability to deliver the performance needed to make all of its capacity usable. What the model adds is a way to link both views. The effective cost of a given disk technology depends in part upon its performance and also in part upon the applications that it enables. References 127 References 1 J. Voldman . Madelbrot . Hoevel J. Knight P. Rosenfeld Fractal Nature of Software-Cache Interaction IBM Journal of Research and Development
