Re: [PATCH RFC] migration: warn about non-migratable configurations unless '--no-migration' was specified

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

On Mon, Apr 19, 2021 at 06:15:56PM +0100, Daniel P. Berrangé wrote:
> On Mon, Apr 19, 2021 at 06:11:47PM +0100, Dr. David Alan Gilbert wrote:
> > * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > > I would make live migration policy an enum, just to make sure
> > > we are explicit about the requirements:
> > > 
> > > - UNKNOWN: this is the current state in QEMU 6.0, where we don't
> > >   really know what the user expects.
> > >   This can be the default on existing versioned machine types,
> > >   just for compatibility.
> > >   I suggest making this print warnings for every migration
> > >   blocker (like this patch does).
> > >   I suggest deprecating this behavior as soon as we can.
> > > 
> > > - PREFERRED: try to make the VM migratable when possible, but
> > >   don't print a warning or error out if migration is blocked.
> > >   This seems to be the behavior expected by libvirt today.
> > > 
> > > - NOT_NEEDED: live migration is not needed, and QEMU is free to
> > >   enable features that block live migration or change guest ABI.
> > >   We can probably make this the default on machine types that
> > >   never supported live migration.
> > 
> > I suggest you could do this by adding:
> >   -warn-none-migratable
> >   -no-warn-none-migratable
> > 
> > and then argue about defaults another time.
> 
> If we're going to add new args, lets at least future proof our
> approach with an extensible option that we can wire into QMP
> too later
> 
>   -migratable  none|preferred|required 
> 
> and letting us add extra key/value pairs to tune it if desired.

Having said that, we potentially don't need a dedicated arg if we
just make  'migratable=none|preferred|required' be a property of
the machine type and hook everything off that

> 
> > > - REQUIRED: live migration is required, and adding a migration
> > >   blocker would be a fatal error.
> > >   This is already implemented by --only-migratable.
> > >   I suggest making this the default on versioned machine types
> > >   after a few releases, and after deprecating UNKNOWN.

Regards,
Daniel
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