Messages in this thread | | | Date | Sat, 11 May 2002 22:17:42 +0200 | From | Jens Axboe <> | Subject | Re: O_DIRECT performance impact on 2.4.18 (was: Re: [PATCH] 2.5.14 IDE 56) |
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On Sat, May 11 2002, Gerrit Huizenga wrote: > In message <20020511142434.GA1224@suse.de>, > : Jens Axboe writes: > > On Sat, May 11 2002, Roy Sigurd Karlsbakk wrote: > > > On Friday 10 May 2002 17:55, Linus Torvalds wrote: > > > > On Fri, 10 May 2002, Lincoln Dale wrote: > > > > > so O_DIRECT in 2.4.18 still shows up as a 55% performance hit versus no > > > > > O_DIRECT. anyone have any clues? > > > > > > > > Yes. > > > > > > > > O_DIRECT isn't doing any read-ahead. > > > > > > > > For O_DIRECT to be a win, you need to make it asynchronous. > > > > > > Will the use of O_DIRECT affect disk elevatoring? > > > > No, the I/O scheduler can't even tell whether it's being handed > > O_DIRECT buffers or not. > > We tried disabling the elevator while doing Raw IO with DB2 > a couple of weeks ago. The database performance degraded much
I'm curious how you did this -- did you disable sorting and merging, or just sorting? Merging is pretty essential to getting decent I/O speeds in current kernels.
> more than expected. Disks were FC connected Tritons or SCSI > connected ServerRaid (or both?). Oracle often asks for a patch > to disable the elevator since they believe they can schedule IO > better. We didn't try with Oracle in this case, but DB2 and RAW > IO without and elevator was not a good choice.
Due to excessive queue scan times, lock contention, or just slight waste of cycles?
-- Jens Axboe
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