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Message-ID: <58cb370e0601180825l105d81fbk9a5e57b722255f96@mail.gmail.com>
Date: Wed, 18 Jan 2006 17:25:44 +0100
From: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
To: Phillip Susi <psusi@cfl.rr.com>
Subject: Re: io performance...
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>,
       "Jeff V. Merkey" <jmerkey@wolfmountaingroup.com>,
       Max Waterman <davidmaxwaterman@fastmail.co.uk>,
       linux-kernel@vger.kernel.org
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On 1/18/06, Phillip Susi <psusi@cfl.rr.com> wrote:
> I was going to say, doesn't the kernel set the FUA bit on the write
> request to push important flushes through the disk's write-back cache?
> Like for filesystem journal flushes?

Yes if:
* you have a disk supporting FUA
* you have kernel >= 2.6.16-rc1
* you are using SCSI (this includes libata) driver [ support for IDE driver
  will be merged later when races in changing IDE  settings are fixed ]

Bartlomiej

> Alan Cox wrote:
> > Not always. If you have a cache flush command and the OS knows about
> > using it, or if you don't care if the data gets lost over a power
> > failure (eg /tmp and swap) it makes sense to force it.
> >
> > The raid controller drivers that fake scsi don't always fake enough of
> > scsi to report that they support cache flushes and the like. That
> > doesn't mean the controller itself is neccessarily doing one thing or
> > the other.
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