Re: Write cache rate

On 11/30/05, Doug Conrey <doug_conrey@oci.com> 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. > 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? 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. One system I worked with had a couple hundred disk writes per hour and a lot of read activity; it never worried me in the slightest that the write hit ratio was down in the 10-30% range, and the users never complained of the performance of writes either. -- Jonathan Leffler #include <disclaimer.h> Email: jleffler@earthlink.net, jleffler@us.ibm.com Guardian of DBD::Informix v2005.02 -- http://dbi.perl.org/ sending to informix-list