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Re: [Qemu-devel] [PATCH 3/4] savevm: fix savevm after migration


From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH 3/4] savevm: fix savevm after migration
Date: Tue, 28 Mar 2017 12:13:15 +0100
User-agent: Mutt/1.8.0 (2017-02-23)

* Kevin Wolf (address@hidden) wrote:
> Am 28.03.2017 um 12:55 hat Dr. David Alan Gilbert geschrieben:
> > * Kevin Wolf (address@hidden) wrote:
> > > Am 25.02.2017 um 20:31 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > > After migration all drives are inactive and savevm will fail with
> > > > 
> > > > qemu-kvm: block/io.c:1406: bdrv_co_do_pwritev:
> > > >    Assertion `!(bs->open_flags & 0x0800)' failed.
> > > > 
> > > > Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> > > 
> > > What's the exact state you're in? I tried to reproduce this, but just
> > > doing a live migration and then savevm on the destination works fine for
> > > me.
> > > 
> > > Hm... Or do you mean on the source? In that case, I think the operation
> > > must fail, but of course more gracefully than now.
> > > 
> > > Actually, the question that you're asking implicitly here is how the
> > > source qemu process should be "reactivated" after a failed migration.
> > > Currently, as far as I know, this is only with issuing a "cont" command.
> > > It might make sense to provide a way to get control without resuming the
> > > VM, but I doubt that adding automatic resume to every QMP command is the
> > > right way to achieve it.
> > > 
> > > Dave, Juan, what do you think?
> > 
> > I'd only ever really thought of 'cont' or retrying the migration.
> > However, it does make sense to me that you might want to do a savevm
> > instead; if you can't migrate then perhaps a savevm is the best you
> > can do before your machine dies.  Are there any other things that
> > should be allowed?
> 
> I think we need to ask the other way round: Any reason _not_ to allow
> certain operations that you can normally perform on a stopped VM?
> 
> > We would want to be careful not to accidentally reactivate the disks
> > on the source after what was actually a succesful migration.
> 
> Yes, that's exactly my concern, even with savevm. That's why I suggested
> we could have a 'cont'-like thing that just gets back control of the
> images and moves into the normal paused state, but doesn't immediately
> resume the actual VM.

OK, lets say we had that block-reactivate (for want of a better name),
how would we stop everything asserting if the user tried to do it 
before they'd run block-reactivate?

Dave

> Kevin
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK



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