Jens Axboe wrote: > On Fri, Feb 08 2008, Jens Axboe wrote: >> On Fri, Feb 08 2008, Paul Clements wrote: >>> Andrew Morton wrote: >>>> On Fri, 8 Feb 2008 09:33:41 -0800 Randy Dunlap >>>> wrote: >>>> >>>>> On Fri, 08 Feb 2008 11:47:42 -0500 Paul Clements wrote: >>>>> >>>>>> There have been numerous reports of problems with nbd and cfq. Deadline >>>>>> gives better performance for nbd, anyway, so let's use it by default. >>>> Please define "problems". If it's just "slowness" then we can live with >>>> that, but I'd hope that Jens is aware and that it's understood. >>>> >>>> It it's "hangs" or "oopses" then we panic. >>> The two problems I have experienced (which may already be fixed): >>> >>> 1) nbd hangs with cfq on RHEL 5 (2.6.18) -- this may well have been fixed >>> >>> There's a similar debian bug that has been filed as well: >>> >>> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=447638 >>> >>> 2) nbd performs about 10% better (the last time I tested) with deadline >>> vs. cfq (the overhead of cfq doesn't provide much advantage to nbd [not >>> being a real disk], and you end up going through the I/O scheduler on >>> the nbd server anyway, so it makes sense that deadline is better with nbd) >>> >>> There have been posts to nbd-general mailing list about problems with >>> cfq and nbd also. >> I'm fine with that, it's one of those things we'll do automatically when >> we have some sort of disk profile system setup. Devices without seek >> penalties should not use AS or CFQ. >> >> Asking for a non-existing elevator is not an issue, but it may trigger >> both printks and a switch to another elevator. So if you ask for >> "deadline" and it's modular, you'll get cfq again if it's the default. >> >> Your patch looks bad though, you forget to exit the old elevator. And >> you don't check the return value of elevator_init(). >> >> All in all, your patch definitely needs more work before it can be >> included. Thanks Jens. This one should be better. -- Paul