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Although VPE automatically alleviates many of the performance problems which occur with poorly defined files, you need to correct flagrant violations and problems. This is especially true for large or heavily accessed files. Poorly defined files always have a negative effect on processing, even when VPE reduces the effect through intelligent buffering. For example, a file with excessive index levels requires excessive reads to access a record without VPE. With VPE, the excessive reads are exchanged for excessive accesses to the core buffers. This results in wasted CPU processing time accessing the buffers. Storage is still wasted containing the buffers. In a memory constrained system, the I/O overhead of paging and swapping may equal the cost of saved index reads. There is no substitute for properly tuned systems.
The options specified when a VSAM data set is created have a great effect on both its space usage and overall performance. For some options the defaults are adequate, but for others they are a poor choice. VPE generates warning messages for poorly defined files to aid in optimizing VPE performance. The discussion which follows describes situations which cause VPE to produce these messages and how you can correct the problems by adjusting parameters on the IDCAMS Define Cluster statement. CISZ (Data Component CI Size)
VPE flags all data component CI sizes of less than 2048 bytes. These small CI sizes result in reduced space utilization. Larger data component CI sizes result in improved space utilization. In addition, small data component CI sizes increase the number of physical reads required to process a file sequentially. These small sizes are generally a holdover from 3330 disk devices and channels with speeds of 1.5 Mb-per-second.
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