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Re: [Qemu-devel] [PATCH 0/2] Open disk images with O_DIRECT
From: |
Daniel P. Berrange |
Subject: |
Re: [Qemu-devel] [PATCH 0/2] Open disk images with O_DIRECT |
Date: |
Wed, 28 Nov 2007 15:08:20 +0000 |
User-agent: |
Mutt/1.4.1i |
On Wed, Nov 28, 2007 at 02:34:02PM +0000, Samuel Thibault wrote:
> Daniel P. Berrange, le Wed 28 Nov 2007 14:27:39 +0000, a écrit :
> > On Wed, Nov 28, 2007 at 03:02:50PM +0100, Laurent Vivier wrote:
> > > These patches allow to open file using O_DIRECT and bypass the host I/O
> > > cache.
> > >
> > > [PATCH 1/2] Add "directio" parameter to "-drive"
> > >
> > > Using "directio=on" with "-drive" will open the disk image
> > > file using "O_DIRECT".
> >
> > I don't see the point in adding a config param for this. If it provides a
> > useful performance improvement (or other benefit) we should enable it by
> > default all the time.
>
> That depends on the mileage of the user. Sometimes it is useful since
> it avoids the duplication of page or buffer cache between the guest and
> the host, sometimes it is not because the guest is not so i/o friendly
> and hence using the host page/buffer cache is useful.
I don't buy that - all OS already do I/O caching because its useful even
on baremetal. As an end-user how do you decide whether to turn on the
directoio=on/off option or not ? Most people won't notice it & will just
run with the default setting - those who do notice are just subjected to
trial-and-error to figure out whether the setting is any use. IMHO, QEMU
should just pick the best setting.
Dan.
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