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Re: [Qemu-devel] [PATCH] migration: Don't activate block devices if usin

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] migration: Don't activate block devices if using -S
Date: Wed, 11 Apr 2018 14:49:45 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

Am 11.04.2018 um 12:01 hat Jiri Denemark geschrieben:
> On Tue, Apr 10, 2018 at 16:47:56 +0200, Kevin Wolf wrote:
> > Am 10.04.2018 um 16:22 hat Dr. David Alan Gilbert geschrieben:
> > > * Kevin Wolf (address@hidden) wrote:
> > > > Am 10.04.2018 um 12:40 hat Dr. David Alan Gilbert geschrieben:
> > > > > Hmm; having chatted to Jiri I'm OK with reverting it, on the condition
> > > > > that I actually understand how this alternative would work first.
> > > > > 
> > > > > I can't currently see how a block-inactivate would be used.
> > > > > I also can't see how a block-activate unless it's also with the
> > > > > change that you're asking to revert.
> > > > > 
> > > > > Can you explain the way you see it working?
> > > > 
> > > > The key is making the delayed activation of block devices (and probably
> > > > delayed announcement of NICs? - you didn't answer that part) optional
> > > > instead of making it the default.
> > > 
> > > NIC announcments are broken in similar but slightly different ways;  we
> > > did have a series on list to help a while ago but it never got merged;
> > > I'd like to keep that mess separate.
> > 
> > Okay. I just thought that it would make sense to have clear migration
> > phases that are the same for all external resources that the QEMU
> > processes use.
> I don't think NIC announcements should be delayed in this specific case
> since we're dealing with a failure recovery which should be rare in
> comparison to successful migration when we want NIC announcements to be
> send early. In other words, any NIC issues should be solved separately
> and Laine would likely be a better person for discussing them since he
> has a broader knowledge of all the fancy network stuff which libvirt
> needs to coordinate with.

Well, if I were the migration maintainer, I would insist on a properly
designed phase model that solves the problem once and for all because it
would be clear where everything belongs. We could still have bugs in the
future, but that would be internal implementation bugs with no effect on
the API.

But I'm not the maintainer and Dave prefers to deal with it basically as
a bunch of one-off fixes, and that will work, too. It will probably
clutter up the external API a bit (because the management tool will have
to separately address migration of block devices, network devices and
possibly other things in the future), but that shouldn't matter much for
libvirt. Maybe what we do need is some documentation of the recommended
process for performing a live migration so that management tools know
which QMP commands they need to issue when.


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