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Re: [Qemu-devel] client_migrate_info - do we need a new command?

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] client_migrate_info - do we need a new command?
Date: Thu, 15 Dec 2011 10:03:54 +0100
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20111104 Red Hat/3.1.16-2.el6_1 Thunderbird/3.1.16


> Originally, the command was a normal sync command and my understanding
> is that it simply posted notification to the clients.  Apparently, users
> of the interface need to actually know when the client has Ack'd this
> operation because otherwise it's racy since a disconnect may occur
> before the client processes the redirection.

No.  The problem is that qemu doesn't process any other I/O while a
incoming migration is running, thats why we have to serialize things:
First have spice client connect to the target (and wait for that op to
finish).  Then kick off live migration.

That reminds me that there is another way to fix it:  Simply lift the
restriction to not process I/O while the incoming live migration runs,
then the need to serialize goes away.

What is the status here?  I remember this being discussed in the past
for other reasons.  Also moving migration to a thread would probably
easily allow the I/O thread run in parallel ...

> The trouble with using a async command for this is that the time between
> (1) & (2) may be arbitrarily long.  Since most QMP clients today always
> use a NULL tag, that effectively means the monitor is blocked for an
> arbitrarily long time while this operation is in flight.

There is a pretty short timeout (five seconds or so).


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