Re: [Qemu-devel] [PATCH 0/2] Open disk images with O_DIRECT

["Daniel P. Berrange" <berrange@redhat.com>]

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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