Re: [PATCH v2] block/file-posix: Optimize for macOS

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

On Tue, Mar 09, 2021 at 10:37:18AM +0100, Kevin Wolf wrote:
> Am 09.03.2021 um 05:52 hat Akihiko Odaki geschrieben:
> > 2021年3月9日(火) 0:37 Akihiko Odaki <akihiko.odaki@gmail.com>:
> > >
> > > 2021年3月9日(火) 0:17 Stefan Hajnoczi <stefanha@redhat.com>:
> > > >
> > > > The live migration compatibility issue is still present. Migrating to
> > > > another host might not work if the block limits are different.
> > > >
> > > > Here is an idea for solving it:
> > > >
> > > > Modify include/hw/block/block.h:DEFINE_BLOCK_PROPERTIES_BASE() to
> > > > support a new value called "host". The default behavior remains
> > > > unchanged for live migration compatibility but now you can use "host" if
> > > > you know it's okay but don't care about migration compatibility.
> > > >
> > > > The downside to this approach is that users must explicitly say
> > > > something like --drive ...,opt_io_size=host. But it's still better than
> > > > the situation we have today where user must manually enter values for
> > > > their disk.
> > > >
> > > > Does this sound okay to everyone?
> > > >
> > > > Stefan
> > >
> > > I wonder how that change affects other block drivers implementing
> > > bdrv_probe_blocksizes. As far as I know, the values they report are
> > > already used by default, which is contrary to the default not being
> > > "host".
> > >
> > > Regards,
> > > Akihiko Odaki
> > 
> > Let me suggest a variant of Stefan's approach:
> > 
> > Modify include/hw/block/block.h:DEFINE_BLOCK_PROPERTIES_BASE() to
> > support a new value called "host". The default values for block size
> > properties may be "host" or not, but they should be consistent. If
> > they are "host" by default
> 
> I'm not sure if it's a good idea, but maybe we could make it so that the
> default is "host" only as long as you didn't specify -nodefaults? Then
> libvirt would automatically keep the old behaviour (because it always
> sets -nodefaults) and manual invocations would usually get the new one.
> 
> Of course, when I start with "I'm not sure if it's a good idea", it's
> usually not, but I wanted to share the thought anyway...

Can you elaborate on what the actual live migration problem is, and
its impact ?  This patch is touching the block backends, so I'm
wondering how backend data ends up having an impact on the migration
stream which is all frontend device data ?  I'm especially concerned
by the mention that some block backends already have this problem,
and wondering if it already impacts libvirt ?

Using -nodefaults is good practice, but I'm still uncomfortable saying
that its use is a requirement if you want migration to work, as that
feels like a change in semantics for non-libvirt users (who can be
mgmt apps, nor merely human interactive users).

Regards,
Daniel
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