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[Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events
From: |
Daniel P. Berrange |
Subject: |
[Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events |
Date: |
Wed, 26 May 2010 16:15:42 +0100 |
User-agent: |
Mutt/1.4.1i |
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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- Re: [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, (continued)
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Juan Quintela, 2010/05/25
- Re: [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Daniel P. Berrange, 2010/05/25
- Re: [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Anthony Liguori, 2010/05/25
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Juan Quintela, 2010/05/25
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Daniel P. Berrange, 2010/05/26
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Anthony Liguori, 2010/05/26
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events,
Daniel P. Berrange <=
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Anthony Liguori, 2010/05/26
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Luiz Capitulino, 2010/05/27
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Juan Quintela, 2010/05/27
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Luiz Capitulino, 2010/05/27
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Anthony Liguori, 2010/05/27
- Re: [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Daniel P. Berrange, 2010/05/26
- Re: [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Luiz Capitulino, 2010/05/25
- [Qemu-devel] Re: [PATCH 3/5] QMP: Introduce MIGRATION events, Juan Quintela, 2010/05/25
- Re: [Qemu-devel] [PATCH 3/5] QMP: Introduce MIGRATION events, Daniel P. Berrange, 2010/05/25
Re: [Qemu-devel] [PATCH 3/5] QMP: Introduce MIGRATION events, Luiz Capitulino, 2010/05/25