Re: [Qemu-devel] KVM call agenda for October 25

[Anthony Liguori]

On 10/25/2011 08:56 AM, Kevin Wolf wrote:
> Am 25.10.2011 15:05, schrieb Anthony Liguori:
> > On 10/25/2011 07:35 AM, Kevin Wolf wrote:
> > > Am 24.10.2011 13:35, schrieb Paolo Bonzini:
> > > > On 10/24/2011 01:04 PM, Juan Quintela wrote:
> > > > > Hi
> > > > >
> > > > > Please send in any agenda items you are interested in covering.
> > > >
> > > > - What's left to merge for 1.0.
> > >
> > > I would still like to cache the default cache mode (probably to
> > > cache=writeback). We don't allow guests to toggle WCE yet which Anthony
> > > would have liked to see before doing the change. Is it a strict requirement?
> >
> > I don't see a way around it.  If the default mode is cache=writeback, then we're
> > open to data corruption in any guest where barrier=0.  With guest togglable WCE,
> > it ends up being a guest configuration issue so we can more or less defer
> > responsibility.
>
> So do you think that offering a WCE inside the guest would be a real
> solution or just a way to have an excuse?

No, it offers a mechanism to "fix mistakes" at run-time verses at start up time. 
 It also means that you can make template images that understand that they 
don't support barriers and change the WCE setting appropriately.

> Christoph said that OSes don't usually change this by themselves, it
> would need an administrator manually changing the setting. But if we
> require that, we can just as well require that the administrator set
> cache=writethrough on the qemu command line.

The administrator of the guest != the administrator of the host.

> > Do you think it's a good idea to change the default mode w/o guest WCE toggle
> > support?  What's your view about older guests if we change the default mode?
> > What's your main motivation for wanting to change the default mode?
>
> Because people are constantly complaining about the awful
> (cache=writethrough) performance they get before they are told they
> should use a different cache option. And they are right. The
> out-of-the-box experience with qemu's block performance really sucks.

With qcow2 you mean, right?

> > I'd be much more open to changing the default mode to cache=none FWIW since the
> > risk of data loss there is much, much lower.
>
> I think people said that they'd rather not have cache=none as default
> because O_DIRECT doesn't work everywhere.

Where doesn't it work these days?  I know it doesn't work on tmpfs.  I know it 
works on ext[234], btrfs, nfs.

Regards,

Anthony Liguori

> Kevin