RE: Write cache rate

Doug Conrey wrote: > I have a question about tuning the write cache %. I have an OLTP > database running IDS 9.40 on AIX 5.2. There are 300,000 buffers > configured (about 30% of physical memory -- 4 GB), and the read cache > rate is pretty consistently 98.5 - 99.0%, but the write cache rate > hovers between 70 and 75%, which seems low to me. Jonathan Leffler replied: > Not everybody agrees with me, but in my view, if the total number of > writes is much less than the total number of reads - which is usually > the case - the write hit ratio isn't all that important. The only > issue is 'how much less' - and it depends on your system. I frequently agree with Jonathan. In this case, he makes the key comment, "it depends on your system." My company provides an application with Informix embedded to about 300 customers. The app is primarily OLTP but with some DSS components in the same database. We have done quite a bit of tuning at various sites, because each has unique requirements. Most of our customers, though, find that, by the time they have finished tuning their system to correctly get the read cache rate high, the write cache rate is down where yours is. Trying to tune the write cache rate is usually futile. Generally, it takes too much DBA effort for too little performance gain, not to mention the frustration of starting and stopping IDS. So, your instinct was right that you "don't want to twiddle settings that are unlikely to help." However, it is this whole symptom which may be a non-issue for you. The read cache rate has far greater user impact than the write cache rate, especially on a system which does a lot of inserts. That said, Malcolm made a good recommendation with Art Kagel's calculations. Having your RA_PAGES or BUFFERS miscalculated could have other performance impact which your users may be feeling, besides the read cache rate. The nice thing is that these tools can help you make the decision, yourself. As I said, our application is mixed OLTP and DSS. This is why some of our customers frequently need BUFFERS set to 25-30% of the physical memory in order for nightly reports and data mining type operations, though, during the day, when more OLTP activity is going on, this is too much. Since the instance is 24*7, they use the same onconfig all day long. Sincerely, Christopher Coleman Steering Committee President Kansas City Informix Users Group www.iiug.org/kciug Database Analyst Pharmacy Division Mediware Information Systems, Inc. sending to informix-list