Re: Good Buffer Size For Tuning Informix?

Chris Jenkins (chrisj@informix.com) wrote: : The only really important factor in determining the number of buffers is : the read and write cache hit ratios reported by tbstat -p. Keep : increasing the number of buffers until these cache hit rates do not : increase when the system is running a 'normal' workload. This number : of buffers is optimum for the workload. Having significantly more : buffers than this optimum simply wastes valuable memory. Forget write cache ratios - they are completely meaningless. ALL writes in online are cached; a page that is modified is always going to be resident in the buffer pool, even if it means bringing the page in for the write. What the write cache is giving you is the relationship between how often you modified pages and how often you flush dirty pages in the buffer pool. For an OLTP system with high throughput, you want to keep your LRU max and min DIRTY values low, which means you will be writing pages fairly often (but your checkpoints will be quicker as a result). That results in a lower write cache, but who cares? Since writes are completely asynchronous to database activity, they have (generally) little appreciable impact. Be careful when monitoring read cache hits as well; you want to zero out your stats when doing this (tbstat -z). Modest improvements get lost in the noise when the stats are for long time periods. Finally, the value of increased buffer pool sizes depends a lot on the nature of the app. For example, in a DSS environment, using OnLine 7.10 or better, the buffer pool may get very little use, if light scans are being used to read data. In that case, you'd be better off increasing the DS memory tuneable than dedicating the memory to the buffer pool. For OLTP, though, the buffer pool is a good bet. Dave Kosenko Disclaimer: All opinions expressed in this message are well-reasoned and insightful; needless to say, they are not those of Informix Software, its partners or lackeys. Anyone who says otherwise is itching for a fight. **************************************************************************** "I look back with some satisfaction on what an idiot I was when I was 25, but when I do that, I'm assuming I'm no longer an idiot." - Andy Rooney