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. > Since this is a sensitive server, I don't want to twiddle settings that > are unlikely to help. I'm not sure it's a good idea to just throw more > buffers at the thing, since it's got lots. I didn't configure the > server, so I'm not certain what the thinking was behind some of the > configurations. I do see that the RA_PAGES is set to 1520, which seems > pretty likely to push things out. Yes excessive read-ahead will definitely reduce the read and write cache efficiency. I have never found a good reason, except for using JBOD, for any but very small RA_ values. I use 16 & 8 and it works VERY well. With the drives, controllers, and RAID arrays all caching read-aheads for you there's no reason to sacrifice buffers for read-ahead. Other reasons for low write cache: excessively small LRU_MIN/MAX_DIRTY values (for pure OLTP and a very large cache you may have to sacrifice here until you get to using 9.40/10.00 with fractional values for these. Art S. Kagel > What is the most likely way to improve the write performance? I feel > like the read-ahead is pulling too much data (though onstat -p shows it > to be using nearly all of it). Is there another setting likely to > improve this? > Thanks. > > DC > sending to informix-list sending to informix-list