Re: [Qemu-devel] Re: [patch 2/3] Add support for live block copy

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/28/2011 02:38 AM, Avi Kivity wrote:
>>
>> We don't separate configuration from guest state today.  Instead of 
>> setting ourselves up for failure by setting an unrealistic standard 
>> that we try to achieve and never do, let's embrace the system that is 
>> working for us today.  We are authoritative for everything and guest 
>> state is intimately tied to the virtual machine configuration.
>
> "we are authoritative for everything" is a clean break from everything 
> that's being done today.  It's also a clean break from the model of 
> central management plus database.  We can't force it on people.

No, it isn't.  This has been the way QEMU has always been.

QEMU has always and will always continue to be useful in the absence of 
a management layer.  That means that it will mix modifications to 
configuration with guest actions.

To avoid races, a management tool needs to either poll QEMU or listen 
for events to know when QEMU initiates a configuration change.  This is 
how tools are written today.

The only thing being discussed is how to handle a very small and rare 
race condition whereas QEMU may send an notification to a crashed 
management tool *and* QEMU crashes before the management tool restarts.

The only two options to solve this problem are synchronous notifications 
or a QEMU global state file.  Since the former is a radical change to 
our existing API, the later is a much more reasonable option.

If a management tool doesn't care about this race, it can simply not use 
the global state file.

Regards,

Anthony Liguori