[Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events

[Daniel P. Berrange]

On Wed, May 26, 2010 at 09:54:22AM -0500, Anthony Liguori wrote:
> On 05/26/2010 05:33 AM, Daniel P. Berrange wrote:
> >>>I'm not sure why you would need a notification of when migration
> >>>starts (since you know when you've started migration).
> >>>       
> >>But you don't know if the other end "knows" that it has also started.
> >>
> >>started is needed only in incoming part, because .... we don't have a
> >>monitor to ask if migration has started.
> >>     
> >If we ever want to get closer to allowing multiple monitors, or allowing
> >apps to issue QMP commands directly via libvirt, then we still need the
> >'migration started' event on the source, because something else can
> >have issued the 'migrate' command without the mgmt app knowing.
> >   
> 
> Migration started doesn't help multiple monitors.  You need locking of 
> some sort.
> 
> Part of the problem is the QMP migrate command is implemented as a 
> synchronous command.  It really ought to be an asynchronous command.  
> That tells you when the migration has actually completed without polling.

Handling asynchronous commands is alot more complicated and error 
prone for client apps, than providing a asynchronous event notification
of the lifecycle stages. If you want to also query status while waiting
for the completion, it means you can have to deal with overlapping 
command  execute+return pairs within a single monitor connection.
AFAICT this requires a change to QMP to require a unique ID to be 
sent with the {'execute'..} command and be sent back with the later
corresponding {'return'...} data,  so you can actually correlate 
reliably.

> On the destination side, we're really limited by the fact that we don't 
> do live incoming migrations.  The monitor doesn't get a chance to run at 
> all with exec: migration, for instance.

If QEMU let you issue a monitor command for starting incoming
migration, instead of using -incoming this wouldn't such a bad
problem. eg you can launch QEMU in the desired config, with CPUs
stopped, do the normal QMP handshake + whatever else is required
then issue 'migrate_incoming URI' which blocked the caller for
the duration, to allow completion to be detected.

> For tcp: and unix:, a CONNECTED event absolutely makes sense (every 
> socket server should emit a CONNECTED event).  Unfortunately, after 
> CONNECTED you lose the monitor until migration is complete.  If 
> something bad happens, you have to exit qemu so once the monitor 
> returns, migration has completed successfully.
> 
> If we introduce live incoming migration, we'll need to rethink things.  
> I would actually suggest that we deprecate the incoming command if we do 
> that and make incoming migration a monitor command.  I would think it 
> should have the same semantics as migrate (as an asynchronous command).  
> A CONNECTED event still makes sense for tcp and unix protocols but I 
> don't think events make sense for start stop vs. an asynchronous command 
> completion.

Do you actually mean 'deprecate -incoming arg' here ? 

Daniel
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