Re: What is good number for bufwaits?

Colburn wrote: > > I just read the latest Informix TechNotes and they suggest bufwaits > should be 0. Mine is at 136,923 (server has only been up 7 hours). My > buffers are at 50,000 for 75 users on a 2Gb database on IUS 9.13. My > read a write cache rates are 99% & 95%. My readaheads add up to my > RS_pagesused. The only thing I could find was an article stating that > it might be too small a difference between RA_PAGES (set to 8) and > RA_THRESHOLD (was 4, I set to 1 last night). My change did not seem to > make a difference. Is this number excessive? If so, what else should I > look at? First, bufwaits is NEVER zero for a REAL working server. That said it is desireable to keep bufwaits to a minimum. A rule of thumb I use is to keep bufwaits to 2-10% of either dskreads or bufwrits (or <3% of the total), since both of these operations require locking an LRU and are most likely to cause a bufwait. There several things that affect bufwaits: 1) More buffers == fewer bufwaits but with your cache %ages this seems OK. 2) More LRUS == fewer bufwaits. This is critical! The more LRUS the better. Avoid LRUS=64 and LRUS=96 as there seems to be a bug in the LRU hash algorithm that causes bufwaits to skyrocket at these particular values (and LRUS=128 triggers a harmless but scary bug at engine startup that complains about zero length Physical and Logical log files). I know that the following values are fine: 4, 16, 32, 127. 3) Lowering LRU_MAX_DIRTY and LRU_MIN_DIRTY can insure that there are sufficient clean buffers available when needed. You can see if this is the problem by looking at FG Writes on the onstat -F report. Foreground should be zero or a fraction of a percent of the total buffer writes. If not you need to be flushing more frequently or you need more buffers, again you buffer count looks fine. Art S. Kagel