Re[2]: Read Ahead Pages, etc.

Art, I find that the RA_PAGES is dependant upon the type of application more than the CPU/DISK performance. For a DSS app where you do lots of sequential scans you want a high value. For a OLTP app a small RA_PAGES is needed. As I stated in a posting yesterday my rule of thumb is to get the read ahead usage to be somewhere between 5% and 10%. The way I achieve this is by determining the type of app, setting a high figure and then do a binary split to get a good value ie: set it at 48 if to high set at 24 etc. I never set the value over 48. My feeling is that to read more from the disk will take too long. If you can find out the maximum kilobytes that can be read from disk in one physical read operation that would probably be a good figure to use for your RA_PAGES setting. Regards David Ashby ______________________________ Reply Separator _________________________________ Subject: Re: Read Ahead Pages, etc. Author: <kagel@bloomberg.com> at WCA-INET Date: 14/5/98 2:25 PM Andrew Lawrenson wrote: > > Is there any optimum upper limit for the values of RA_PAGES and > RA_THRESHOLD? > I've bumped mine up from the defaults of 4/2 to 64/16, and I'm still at > 99.9% pages used. How high should I go? Should I just keep going until the > percentages fall? Yes, you can keep increasing the values as long as you still gain something from the effort as indicated by the near 100% RA utilization. You can also look at your read cache% if that has been falling then even though you are taking advantage of the read ahead you may be bumping other pages out of the buffer cache so that the read ahead is being used to refetch pages that were bumped by other read ahead but were still needed. This is the trade off. To double check see if the read aheads fall off when you add more buffers. BTW just for curiosity, the only time I have seen such high RA requirements be truly effective was where the CPUs were very fast but the disk farm relatively slow. Are you using singleton drives or RAID/ Stripping? Are you using OS, Informix, or hardware mirrors? How many controllers are the drives hung on? Are these SCSI, SCSI2, Ultra SCSI, Ultra Wide SCSI, IDE, EIDE, or SSA drives? What is the spindle speed? What speed/type CPUs do you have? How many? I am writing an article for TechNotes on engine tuning (for issue 3) and this is one area where my own experiences are not broad enough. Anyone else who wants to send me details of their equipment and RA conditions are also welcome. We can help the entire Informix community with RA tuning if we share. Art S. Kagel