The Past.....

4 Buffer Pools
- 3 4K Pools
- 1 32K Pool
Fixed Thresholds
- DEATH Out of Space 100%
- IWTH Immediate Write Threshold 97.5%
- DMTH DataManager Threshold 95%
- SPTH Sequential Prefetch Threshold 90%
- DWTH Deferred Write Threshold 50%
- VDWTH Vertical Deferred Write Threshold 10% per DS
  - No Counter of its Own, Uses DWTH Counter
- VPSEQT % of Pool Available for Prefetch Pages 80%
Somewhat Limited Instrumentation

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Notes:

Prior to V3, we were quite limited in our tuning ability because we only had three 4K pools to separate the objects. Many installations maintained an erroneous assumption that they could get better performance with one large pool rather than multiple pools. Several studies showed that installations using as few as 6000 buffers, for BP0 (only), obtained markedly better performance when using at least two pools and the same number of buffers.

Several major misconceptions existed regarding buffer pool performance, many of them fostered by inaccurate definitional information provided by online monitors and other documentation. The DWTH (deferred write threshold) is incremented in every system - however, in most cases this is really the VDWTH threshold at the object level that did not have its own counter. Current Buffers Active (CBA) is shown in most monitors as ‘amount of pool in use’; this is incorrect, since 100% of the pool is ALWAYS used. CBA is the amount of the pool that is ‘not available’ because the buffers contain updated and uncommitted data, or are currently having data read into them. A Getpage/Read I/O ratio is a very poor measure of pool efficiency. A true pool hit ratio provides a more accurate measurement and is covered later in this presentation.