tailieunhanh - THE FRACTAL STRUCTURE OF DATA REFERENCE- P19

THE FRACTAL STRUCTURE OF DATA REFERENCE- P19: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. | 78 THE FRACTAL STRUCTURE OF DATA REFERENCE utilizations the linear model can apply only if some segments are held in reserve. By there is no other way to achieve an average segment utilization outside the range of 5-100 percent. 3. IMPACT OF TRANSIENT DATA ACCESS Returning to the dormitory analogy we have just assumed in the preceding analysis that students drop out at a constant rate. This assumption is not very realistic however. We should more correctly anticipate a larger number of students to drop out in the first term than in subsequent terms. Similarly once a fresh data item is written into a segment we should expect due to transient data access that the probability of further updates is highest shortly afterward. Figure . Distribution oftime between track updates for the user and system storage pools also presented in Figure . The hierarchical reuse model provides the ideal mathematical device with which to examine this effect. To do so we need merely proceed by assuming that applies not only to successive data item references in general but also to successive writes. Figure helps tojustify this assumption. It presents the distribution of interarrival times between writes for the same vm user and system storage pools that we first examined in Chapter 1. Note in comparing Figure writes with Figure all references that a small difference in slopes is apparent say 0 for writes as contrasted with 0 for all references . Despite Figure the application ofthe hierarchical reuse model to free space collection does represent something of a leap of faith . The time scales relevant to free space collection are much longer than those presented in Figure Free Space Collection in a Log 79 . The appropriate time scales would extend from a few minutes up to several days or weeks. Nevertheless the hierarchical reuse model greatly improves the realism of our previous analysis. We need no longer assume that data items are rendered .