THE FRACTAL STRUCTURE OF DATA REFERENCE- P21: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. | Transient and Persistent Data Access 89 window lengths much longer than the time needed to complete a typical burst one might usually expect to see an entire burst isolated somewhere in the window. But for window lengths shorter than this characteristic time and assuming that such a characteristic time actually exists one might usually expect to see some portion of a burst spread throughout the window. The characteristic burst time and the corresponding number ofrequests could then be identified by the transition between these two patterns ofbehavior. The result is in sharp contrast however with the outcome of the thought experiment just presented. Patterns of reference that conform to the hierarchical reuse model although bursty do not preferentially exhibit bursts ofany specific quantifiable length. Instead they are bursty at all time scales. It is reasonable to hope therefore that individual transient or persistent data items can be distinguished from each other by applying the metric . Provided that the time window is long enough for the persistence of a given item of data to become apparent this persistence should be reflected in a high outcome for P. On the other hand regardless of the time scale we may hope to recognize transient data from a small outcome for P. Two i o traces each covering 24 hours at a moderate-to-large os 390 installation were used to investigate this idea. The two traced installations were A. A large data base installation running a mix of cics ims DB2 and batch. B. A moderate-sized DB2 installation running primarily on-line and batch DB2 work with on-line access occurring from a number oftime zones in different parts of the world. Figure . Distribution of probability density for the metric P track image granularity. 90 THE FRACTAL STRUCTURE OF DATA REFERENCE Figure . Distribution ofprobability density forthe metric P cylinder image granularity. Time spanned by file I O hours Figure . Distribution of probability density
