Greg Smith wrote:
On Tue, 6 May 2008, Craig James wrote:
I only did two runs of each, which took about 24 minutes. Like the
first round of tests, the "noise" in the measurements (about 10%)
exceeds the difference between scheduler-algorithm performance, except
that "anticipatory" seems to be measurably slower.
Those are much better results. Any test that says anticipatory is
anything other than useless for database system use with a good
controller I presume is broken, so that's how I know you're in the right
ballpark now but weren't before.
In order to actually get some useful data out of the noise that is
pgbench, you need a lot more measurements of longer runs. As
perspective, the last time I did something in this area, in order to get
enough data to get a clear picture I ran tests for 12 hours. I'm hoping
to repeat that soon with some more common hardware that gives useful
results I can give out.
This data is good enough for what I'm doing. There were reports from non-RAID users that the I/O scheduling could make as much as a 4x difference in performance (which makes sense for non-RAID), but these tests show me that three of the four I/O schedulers are within 10% of each other. Since this matches my intuition of how battery-backed RAID will work, I'm satisfied. If our servers get overloaded to the point where 10% matters, then I need a much more dramatic solution, like faster machines or more machines.
Craig
