Re: a heavy duty operation on an "unused" table kills my server

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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

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