From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=51432 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OHdSA-0004qz-Ie for qemu-devel@nongnu.org; Thu, 27 May 2010 09:48:55 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OHdS8-0001dS-SE for qemu-devel@nongnu.org; Thu, 27 May 2010 09:48:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34758) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OHdS8-0001d9-JZ for qemu-devel@nongnu.org; Thu, 27 May 2010 09:48:52 -0400 Date: Thu, 27 May 2010 10:48:45 -0300 From: Luiz Capitulino Message-ID: <20100527104845.341fa9e5@redhat.com> In-Reply-To: <4BFD5283.70809@codemonkey.ws> References: <9b6575587d22a5c85ec536172810520ee3b945d5.1274796992.git.quintela@redhat.com> <4BFBE843.5070202@codemonkey.ws> <4BFBF36D.8070208@codemonkey.ws> <20100525162549.GQ31759@redhat.com> <4BFBFBCB.2070806@codemonkey.ws> <20100526103346.GK18547@redhat.com> <4BFD361E.8070708@codemonkey.ws> <20100526151542.GU18547@redhat.com> <4BFD5283.70809@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: qemu-devel@nongnu.org, Juan Quintela On Wed, 26 May 2010 11:55:31 -0500 Anthony Liguori wrote: > On 05/26/2010 10:15 AM, Daniel P. Berrange wrote: > > 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. > > > > 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. [...] > >> 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?