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

[Juan Quintela <quintela@redhat.com>]

Luiz Capitulino <lcapitulino@redhat.com> wrote:
> On Wed, 26 May 2010 11:55:31 -0500
> Anthony Liguori <anthony@codemonkey.ws> wrote:

>> That's exactly how the protocol is designed.  That was one of the major 
>> improvements of QMP over the human monior.
>
>  Yes and it already has 'id' support:
>
> { "execute": "cont", "id": "luiz" }
> {"timestamp": {"seconds": 1274966635, "microseconds": 776813}, "event": "RESUME"}
> {"return": {}, "id": "luiz"}
>
>  But it doesn't detect duplicates, this is something I think it's up
> to the client to do, do you agree?
>
>> This is how the info balloon command works, BTW.
>
>  I won't remember the details now, but that interface has some issues and it
> has to be reviewed.
>
>> Since there's a clear correlation between the request and the result of 
>> the request, an asynchronous command is what makes the most sense.  It 
>> eliminates the problem of how to pass QErrors via an event which is one 
>> of the problems with the current event proposal.
>
>  Not exactly, this is a problem with QError not the event proposal. We'll
> have the same issue if we decide to include errno in the migrate errors and
> the problem still exists with the BLOCK_IO_ERROR event.
>
>  That said, I do agree that migrate should be asynchronous. This yet another
> thing we may want to fix before 0.13.

How difficult is that?

> [...]
>
>> >> 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 ?
>> >    
>> 
>> Yes.  And by deprecate, I really mean that -incoming just becomes 
>> syntactic sugar for executing a monitor command immediately.
>
>  But we can't change -incoming itself, since our command-line is supposed
> to be stable, right?
>
>  Also, Juan has said that replacing that arg with a monitor command
> doesn't work, as qemu would have to be started in paused monitor for this
> to work.
>
>  So, what about introducing a -incoming-monitor command, which puts qemu
> in the right state for migration, but requires a migrate_incoming command
> to actually start migration?

this -incoming-monitor is called -S, that should have a long name of
-no-autostart

that is what it does, and what we need for incoming migration as monitor
command.  Nothing new to see here.

Later, Juan.