Re: RA, bugs, and performance

Carlson@WHSmith wrote: > Informix DS 7.23.uc4 > HPUX 10.01 > HP K200 4-way > > Up to this time, I have maintained a RA hit percentage of 90-99%. My RA > variables were set to : > > RA_PAGES 150 > RA_THRESHOLD 75> > While these numbers were a bit high, we were able to take advantage of > most of the pages that were read-ahead. > Wow, that's way too high for even a DSS environment. Rule of thumb I use isto set these to about 32, 16 for a good sized DSS and to about 12,8 for an OLTP envrionment. > As part of my tuning, I was planning to revisit these variables. Where > would be a good trade-off between read-aheads and bufwaits? I've seen > the postings by various people, and took them to heart for the next > round of R&D testing on my test system. > > Unfortunately, bug# 85350 changed all of that. RA parms. don't affect bufwaits, they are caused by not having sufficient cleanbuffers available for a session. Check your cleaner activity, number of buffers, IO contention, LRU_MAX and LRU_MIN settings, and also your checkpoint duration to get an idea of what could be leading to sluggish buffer flusing. You might also want to bump up your AIO VP's, if they are taking a beat. > > > I now have reset these parameters to 0. While I didn't mind retuning > these variables, I hadn't planned on resetting them quite like this. > > Any idea of what the impact to my system would be? We're a mixed > environment, OLTP and DSS batch reporting. I believe RA values are set a default, although I've never set them to 0 :-) Hope that helps. Arun