- System Hit Ratio
(Getpages-SumOfPagesRead)/Getpages– Effect/Meaning of Negative Hit Ratios…
– Effect and setting of VPSEQT 
HP Retrieval Ratio
PagesReadHP/PagesWrittenHP
– Is a low ratio always bad, or can it be all right?
- Pages Written per Write
– Adjust VWQT and DWQT - I/O Rate per Second
– System, Each Pool, Objects
The system hit ratio provides and much better indicator of pool performance than a GP/RIO ratio. The HP Retrieval ratio quickly shows the benefit from using a HP. A low percentage does not immediately mean that there is little benefit – you may also have to calculate how many I/O per second you are saving. In most cases, especially for online workloads, the HPSEQT should be set to a low value to reserve the HP space for the randomly accessed data.
When the pages/write is a low single digit value like 3.1 or 2.2, then the VDWQT should be set to zero (0). This will not cause additional writes, but will avoid flooding the system with writes at CKPT time, or hitting VDWQT or DWQT at higher thresholds. There is an internal limit for writing out data – the pages for a single write cannot span more than 150 RBAs. This is what causes the low pages/write, and there is nothing you can do about it.
Considering both overall system and application performance, the I/O rate/sec is one of the most numbers, does not have a direct relationship to the system hit ratio.