tailieunhanh - THE FRACTAL STRUCTURE OF DATA REFERENCE- P13

THE FRACTAL STRUCTURE OF DATA REFERENCE- P13: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. | 46 THE FRACTAL STRUCTURE OF DATA REFERENCE the average residency time. The adjusted hit ratios are obtained by applying the proportionality relationship expressed by with 0 .25 for example the miss ratio for the erp application is projected as .36 x 262 197 -25 .34. Based upon the projected residency times and hit ratios we may then compute the cache storage requirements as already discussed for the previous table. In this way we obtain an objective of2256 megabytes for the cache size of the target system. If these requirements could be met exactly then we would project an aggregate hit ratio for the three applications of 72 percent. As a final step we must also consider how to round off the computed cache memory requirement of2256 megabytes. Since this requirement is very close to 2 gigabytes we might choose in this case to round down rather than up. Alternatively it would also be reasonable to round up to say 3 gigabytes on the grounds that the additional cache can be used for growth in the workload. To account for the rounding off of cache memory we can apply the proportionality expressed by . Thus after rounding down to 2 gigabytes we would expect the aggregate miss ratio ofthe target system to be .28 x 2048 2256 -25 -75 .29 identical to the current aggregate miss ratio ofthe three applications. If the available performance reporting tools are sufficiently complete it is possible to refine the methods presented in the preceding example. In the figures ofthe example the stage size was assumed to be equal to .04 megabytes a reasonable approximation for most os 390 workloads . The capability to support direct measurements of this quantity has recently been incorporated into some storage controls if such measurements are supported they can be found in the System Measurement Facility smf record type 74 subtype 5. Also in the example we used the total miss ratio as our measure of the percentage of i o s that require more cache memory to be allocated. A .