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VPE's Operating System interfaces receive control during VSAM OPEN and CLOSE. During each VSAM file OPEN, VPE will process each RULE statement looking for a match on job, step, or data set. The first RULE statement which matches is the one which is used for the VSAM file being opened. The sequence of the RULE statements determines how your implementation of VPE performs. Code the most specific RULE statements prior to the more general ones. For example, data set specific RULE statements should appear before step specific ones.
There is no limit placed on the number of RULE statements you can use. However, the more RULE statements you code, the more processing is done by VPE during VSAM file open time. For best results, keep the number of RULE statements below 500.
Because the RULE statements can be coded to only affect certain jobs, steps, or data sets, you can use them to safely experiment with different combinations of parameter values. Then, when you have determined good parameter values which should apply to all data sets by default, you can move the parameter settings to the SYSOPT and VSAM statements.
If the ACB for the VSAM file being opened specifies DIR processing and the file has more than one record in it, VPE normally attempts to implement LSR processing for the file. If you know that the file is actually processed sequentially, FORCEMODE=SEQ forces VPE to implement sequential processing enhancements for the file.
If the VSAM file being opened is empty or has only one record, or if the file's ACB specifies SEQ processing, VPE normally attempts to implement sequential processing enhancements for the file. If you know that the file is loaded by the program and then updated in DIR mode, and if the update is quite large, specify FORCEMODE=LSR to get the greater benefit of LSR enhancements for the file.
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