VSAM Performance Enhancement › Advanced Topics › Optimizing VSAM Performance by Adjusting VSAM Parameters › Tuning VPE's Buffer Size

Tuning VPE's Buffer Size

When you first run VPE on a file, start with the default buffer size and review the performance statistics generated in the VSAM Recommendation Report to determine how effective that size is for your application. Run the job several times, with different buffer sizes. While you are incrementally decreasing the buffer size, you will eventually reach the point where a continued decrease of the buffer pool size for a particular job will cause the percent of I/O requests which are being satisfied from the buffer pool to decrease dramatically. For optimum performance, do not go below this value. While you are incrementally increasing the buffer size, a point will be reached where adding more buffer space does very little for the look-aside ratio. For most applications, a look-aside ratio as high as 90 percent can be obtained on all files. Some applications will get even higher ratios with the addition of more buffers, but beyond the 90 percent point the decrease in run time will usually be small. In a memory-constrained environment, the use of too large a value can cause a performance decrease, due to additional paging in the system.

Buffer size changes affect the number of times your job is swapped out. When it is out, it is not doing anything. If swapping is not going up and run times are still out of proportion with CPU consumption, check the following:

• Excessive Program Loads—Correct excessive loads from STEPLIB or JOBLIB by canceling the JOB after it has run a while and inspecting the use count in the CDEs.
• Excessive OPENS or CLOSES—Correct this problem by performing a GTF trace. This trace identifies both OPEN/CLOSE and LOAD problems.
• Enqueue Conflicts—Examine your dumps or use a third party program to identify enqueue problems.