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Date: Sat, 11 May 2002 22:17:42 +0200
From: Jens Axboe <axboe@suse.de>
To: Gerrit Huizenga <gh@us.ibm.com>
Cc: Roy Sigurd Karlsbakk <roy@karlsbakk.net>,
        Linus Torvalds <torvalds@transmeta.com>, Lincoln Dale <ltd@cisco.com>,
        Andrew Morton <akpm@zip.com.au>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Martin Dalecki <dalecki@evision-ventures.com>,
        Padraig Brady <padraig@antefacto.com>,
        Anton Altaparmakov <aia21@cantab.net>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: O_DIRECT performance impact on 2.4.18 (was: Re: [PATCH] 2.5.14 IDE 56)
Message-ID: <20020511201742.GA1106@suse.de>
In-Reply-To: <20020511142434.GA1224@suse.de> <E176bYP-00040t-00@w-gerrit2>
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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