Re: Read-ahead parameters
I was recently looking at the same parameters and thought it was rather strange with the numbers I came up with. My environment: BUFFERS=50,000 bp_fact=0.50 # random % large_queries=2 # random queries RA_PAGES = ( BUFFERS*bp_fract ) / ( 2*large_queries ) + 2 = 25,000 / 4 + 2 = 6,250 + 2 = 6,252 RA_THRESHOLD = ( BUFFERS*bp_fract ) / ( 2*large_queries ) - 2 = 25,000 / 4 - 2 = 6,500 - 2 = 6248 The RA_PAGES and RA_THRESHOLD values in the above calculation vary slightly <g> from the previously mentioned 64 / 32 or 16 / 4. Any thoughts? I did a several tests with the above values. My original query took 20 minutes to run. With the above changes, the queries takes 15 mintues. My original settings were the defaults. Steve Romankiw vyao@dehavilland.ca wrote: > In "Performance Guide for IDS" book( for IDS V7.30), I found this: > > RA_PAGES = ( BUFFERS*bp_fract ) / ( 2*large_queries ) + 2 > RA_THRESHOLD = ( BUFFERS*bp_fract ) / ( 2*large_queries ) - 2 > > bp_fract is the portion of data buffers to use for large scans that require > read-ahead. If you want to allow large scans to table up to 75 percent of > buffers, bp_fract would be 0.75 > large_queries is the number of concurrent queries that require read-ahead that > you intend to support. > > I setup our testing enviornment according this, a huge number. I am loading data > to setup 20G database. Till now the loading is ok, but checkpoint time is over > 20. > > So I don't know what exactly these parameters should be. > > Vivian > vyao@dehavilland.ca > > Doug McAllister <doug.mcallister@fmr.com> on 05/18/99 04:32:41 PM > > Please respond to Doug McAllister <doug.mcallister@fmr.com> > > To: informix-list@iiug.org > cc: (bcc: Vivian Yao/deHavilland/Bombardier) > Subject: Re: Read-ahead parameters > > I have read somewhere that the max value for read ahead is 32. I do not > remember where and I cannot find a reference to a max value in any doc. > Any references? > > Doug McAllister > Fidelity Investments > > "Carlson@WHSmith" wrote: > > > IDS 7.30.uc7 > > HPUX 10.20 > > > > The bugs are supposed to be gone; after eight months I'm ready to > > reenable read-aheads. > > > > Our original values of RA_PAGES / RA_THRESHOLD were 150 / 75. These > > seemed a bit high based on the postings that I've seen since that time. > > We're an OLTP shop, with batch (report) processing thrown in for good > > measure. All of our queries run on some index; we have no large > > sequential table scans. Our disks are 4GB FW SCSI. > > > > I'm planning to start at values of 64 / 32. Any thoughts, comments, or > > experiences? > > > > John Carlson > > Informix DBA > > WHSmith USA