Re: Read ahead

In article <350D8F6F.1E0A@bloomberg.com>, Art S. Kagel <kagel@bloomberg.com> writes >David Williams wrote: >> >> In article <6efd6m$nu0@bgtnsc03.worldnet.att.net>, Jay Aymond >> <jay@ulife.com> writes >> >Any recomendations on what percentage of buffers should be used for >"ra_pages" >> >and "ra_threshhold" in a system that is mostly OLTP and batch processing? >> > >> > >> Not really, run onstat -p and total the first 3 RA columns at the >> bottom, the total should be clsoe to RA-pages used. > >I'll just add two things. First you can increase the Read Ahead until >the totals that David mentions no longer match then back down a little. >Second, the values that work for you will depend quite a bit on the >types of queries you do (if read cache % is high you can probably use >Read Ahead to good advantage if low your queries are not likely to be >sequential enough to gain from Read Ahead) and on the speed of your >disk farm. If your disks are very fast or you use a large stripe set >you may find that you only need a very small read ahead and a smaller >read ahead threshhold and that increasing either will not only not gain >you much but may increase I/O system calls unneccessarily. Conversely, ?? Large values mean less system calls per 64K transfered. RA_PAGES = 8 means 4 x 16K transfers = 4 calls RA_PAGES = 16 means 2x32K transfers = 2 calls >if you use slower and/or single disks then read ahead is VERY >important. > Surely it always helps as long as the pages are used?? Ideally you want to keep the drives busy whilst avoiding unneccessary I/O. >We use 20-30 disk stripes of 7200RPM and faster FAST-WIDE SCSI 2 disks >and get great performance from read ahead values of 16 and a threshhold >of 4. Likewise at my previous job we used only a 4 drive stripe of >5400 RPM Narrow SCSI-2 drives and needed readahead of 64 and threshhold >of 16 to keep up with a MUCH smaller load. > Have you tried 64 and 16 with the your current setup? Did it help? >Art S. Kagel -- David Williams Maintainer of the Informix FAQ Primary site (Beta Version) http://www.smooth1.demon.co.uk Official site http://www.iiug.org/techinfo/faq/faq_top.html I see you standin', Standin' on your own, It's such a lonely place for you, For you to be If you need a shoulder, Or if you need a friend, I'll be here standing, Until the bitter end... So don't chastise me Or think I, I mean you harm... All I ever wanted Was for you To know that I care