2.5. VSAM rule-of-thumb mode
Before we start, let us make a general statement about VSAM cluster parameters affecting performance and their defaults. You should note that these defaults were established a long time ago, when virtual storage was all below the 16 MB line; central storage size restricted; the DASD was slow, removable, and expensive; and channels were a scarce resource.
Therefore, the defaults are outdated in many cases. The reason IBM does not change the defaults is to maintain compatibility with your existing workload. Compatibility is an important feature protecting your investment. You do not need to throw out your code and hardware when z/OS changes a release. However, keep in mind that these VSAM defaults can be easily overwritten ...
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