Re: [Qemu-devel] Re: [CFR 6/10] cont command

[Daniel P. Berrange]

On Wed, Jun 16, 2010 at 03:11:24PM +0200, Juan Quintela wrote:
> Anthony Liguori <address@hidden> wrote:
> > cont
> > ----
> >
> > Resume emulation.
> >
> > Arguments: None.
> >
> > Example:
> >
> > -> { "execute": "cont" }
> > <- { "return": {} }
> 
> This is related to the commands, not QMP per se:
> 
> Once that we are talking about "cont" command.  There are two cases that
> we need to think of:
> 
> - incoming migration:
> 
> If you start with -incoming foo, and then run "cont" on the monitor
> without having started the migration .... corruption is ensured.

This is why '-incoming' command line arg should die, and be replaced
with a 'incoming' monitor command that would simply not allow 'cont'
to be run until it completed.

For that matter, even with '-incoming' arg on command line we could
refuse to honour 'cont' until the incoming migration had been done.

> - outgoing migration
> 
> After sucessful migration, we can issue "cont" command in source, and
> having source and target running at the same time -> disk corruption
> again.

This doesn't have to mean corruption. eg two machines using cluster-LVM.
The target QEMU is using a writable snapshot of the volume the source
QEMU is using. So you could in fact start the source again and have two
copies of the guest running at once. At the QEMU level I don't think we
should try to force policy of this kind, since it'll prevent people
experimenting with interesting new use cases. There are also soooooooo
many other ways you can  trash your data with multiple hosts. If you
want safe migration, use a management app which adds a level of policy
to protect against stupid decisions

Daniel
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