THE FRACTAL STRUCTURE OF DATA REFERENCE- P5: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. | 6 THE FRACTAL STRUCTURE OF DATA REFERENCE 3. MODEL DEFINITION Eventually this book will present abundant statistical summaries of data reference patterns. As a starting point however let us begin with a single observed pattern of access to a single item of data. Figure presents one such pattern among hundreds of thousands observed in a large production database environment running under os 390. In Figure the horizontal axis is a time line upon which most of the requests are marked. When a group of requests are too closely spaced to distinguish along the time line the second and subsequent requests are displaced vertically to give a zoom of each one s interarrival time with respect to the request before it. A careful examination ofFigure makes it clear that the arrivals are driven by processes operating at several distinct time scales. For example episodes occur repeatedly in which the interarrival time is a matterofa few milliseconds such bursts are separated in turn by interarrival times of many seconds or tens of seconds. Finally the entire sequence is widely separated from any other reference to the data. Ifwe now examine the structure ofdatabase software in an effort to account for data reuse at a variety oftime scales we find that we need not look far. For example data reuse may occur due to repeated requests in the same subroutine different routines called to process the same transaction or multiple transactions needed to carry out some overall task at the user level. The explicitly hierarchical structure of most software provides a simple and compelling ex- Figure . Pattern of requests to an individual track. The vertical axis acts as a zoom to separate groups of references that are too closely spaced to distinguish along a single time line. Hierarchical Reuse Model 7 planation for the apparent presence ofmultiple time scales in reference patterns such as the one presented by Figure . Although the pattern of events might well differ between
