Obviously there is a lot of information provided in these reports, especially the report for key-sequenced data sets. It's probably even more obvious that the information isn't much good unless it can be interpreted and put to good use. Unfortunately, it is impossible to develop absolute guidelines for interpreting all of this information and applying it universally to all data sets. We can, however, point out some things to look for in these reports if you are trying to tune your VSAM environment. We will begin with the simplest report (RRDS) and proceed to the ESDS and KSDS reports.
The one performance item that all three of these reports have in common is the EXTENTS value in the catalog information section. This will tell you how many extents the cluster occupies. A high value will not only impair performance when processing the data set, but it also leads to fragmented storage volumes. If possible, clusters should be archived and restored to consolidate the extents into one contiguous area.
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