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

[Anthony Liguori <anthony@codemonkey.ws>]

On 06/16/2010 11:25 AM, Daniel P. Berrange wrote:
>
>> 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.
>    

If we had an incoming migration command, I think we'd have to think 
careful about it's semantics.  Is it reasonable to allow a machine 
that's otherwise running to do an incoming command?

Regards,

Anthony Liguori

>> - 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
>