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    SubjectRe: O_DIRECT performance impact on 2.4.18 (was: Re: [PATCH] 2.5.14 IDE 56)
    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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