Josh Berkus <josh@agliodbs.com> writes: >> I tested the db files residing on a software RAID-1 composed of 2 IDE >> 7200rpm drives on linux 2.6.12. > > FWIW, most performance-conscious users will be using a SCSI RAID > array.
No worry, I'm not out to squeeze every little juice from a particular installation, which in this case is my home computer. I am interested in automating estimation of a suitable RPC value for a given installation. > Well, it's actually calculating the cost ratio of pulling non-sequential > random *rows* from the db files against pulling sequential blocks. Then running it against the db files should yield better estimation than on sequential pages. >> On dbases smaller (calculated from du <dbase_dir>)than 500M, I got a >> ratio (random over sequential time) of 4.5:1. A 3.0GB dbase has a >> ratio of 10:1. On a 3GB contiguous file, the ratio is about 4:1. > > All of this goes to uphold Tom's general assertion that the default of 4 is > more or less correct Doesn't this show that 4:1 is a pretty optimistic value considering that no long-running db files are fragmentation-free? >but the calculation in which we're using that number is > not. The calculation inside the planner, IOW, how the planner uses the RPC value? Thanks, YS. ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
