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THE FRACTAL STRUCTURE OF DATA REFERENCE- P14

THE FRACTAL STRUCTURE OF DATA REFERENCE- P14: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. | Chapter 4 USE OF MEMORY AT THE I O INTERFACE In the traditional view ofthe memory hierarchy the I O interface forms a key boundary between levels. In this view the level above the i o interface consists of high speed processor memory the level below consists of disk storage which can store far more data but requires physical movement including disk rotation and head positioning as part of data access. The presence of storage control cache in modern storage controls has made for complications in the simple picture just described. Essentially similar semiconductor memory technologies now exist on both sides ofthe I O interface. Since the the early 1980 s increasingly large file buffer areas and increasingly large cache memories have become available. This raises the question of how best to manage the deployment of semiconductor memory some for file buffer areas and some for storage control cache so as to maximize the gains in application performance. The theme of this chapter is that for most application data it is possible to accomplish performance gains through a division oflabor in which each ofthe two memory technologies plays a specific role File buffer areas in the processor are used to hold individual data records for long periods of time. Storage control cache contributes additional hits by staging the entire track of data following a requested record and holding it for shorter times. In addition storage control cache provides the capability to cache writes which usually cannot be hardened in the processor buffers. In broad terms the objective of this strategy is to minimize the number of requests that must be serviced via physical disk access. Equivalently the objective is to minimize the number of storage control cache misses. It is 52 THE FRACTAL STRUCTURE OF DATA REFERENCE important to observe that this is not the same as wishing to maximize the number of storage control cache hits. Instead we choose to service many or most application requests in the .