On Wed, 20 Jan 2010, Greg Smith wrote:
Basically, to an extent, that's right. However, when you get 16 drives or
more into a system, then it starts being an issue.
I guess if I test a system with *only* 16 drives in it one day, maybe I'll
find out.
*Curious* What sorts of systems have you tried so far?
As the graph I just sent shows, the four schedulers are pretty-much
identical in performance, until you start saturating it with simultaneous
requests. CFQ levels out at a performance a little lower than the other
three.
Seriously though, there is some difference between a completely synthetic
test like you noted issues with here, and anything you can see when running
the database.
Granted, this test is rather synthetic. It is testing the rather unusual
case of lots of simultaneous random small requests - more simultaneous
requests than we advise people to run backends on a server. You'd probably
need to get a RAID array a whole lot bigger than 16 drives to have a
"normal workload" capable of demonstrating the performance difference, and
even that isn't particularly major.
Would be interesting research if anyone has a 200-spindle RAID array
hanging around somewhere.
Matthew
--
A good programmer is one who looks both ways before crossing a one-way street.
Considering the quality and quantity of one-way streets in Cambridge, it
should be no surprise that there are so many good programmers there.
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