[PATCH] qemu: migration: don't open storage driver too early

[Daniel P. Berrangé]

On Mon, Oct 12, 2020 at 09:12:50AM -0400, Cole Robinson wrote:
> On 10/12/20 5:05 AM, Daniel P. Berrangé wrote:
> > On Mon, Oct 12, 2020 at 10:55:52AM +0200, Michal Privoznik wrote:
> >> On 10/12/20 10:50 AM, Daniel P. Berrangé wrote:
> >>> On Sun, Oct 11, 2020 at 07:44:36PM -0400, Cole Robinson wrote:
> >>>> If storage migration is requested, and the destination storage does
> >>>> not exist on the remote host, qemu's migration support will call
> >>>> into the libvirt storage driver to precreate the destination storage.
> >>>>
> >>>> The storage driver virConnectPtr is opened too early though, adding
> >>>> an unnecessary dependency on the storage driver for several cases
> >>>> that don't require it. This currently requires kubevirt to install
> >>>> the storage driver even though they aren't actually using it.
> >>>>
> >>>> Push the virGetConnectStorage calls to right before the cases they are
> >>>> actually needed.
> >>>
> >>> This pushes the connection open attempts inside a loop body. So if the
> >>> VM has multiple disks, then we're going to be repeatedly opening and
> >>> closing the connection which is not desirable.
> >>>
> >>> I think we need a global connection across all disks, which is lazy
> >>> opened on first access.
> >>
> >> Doesn't virGetConnectStorage() cache the connection?
> > 
> > Yes & no.
> > 
> > You can call virSetConnectStorage(conn) to populate the cache with a
> > pre-opened connection. If you haven't done that, then a new connection
> > will be opened every time. The ltter is what this code will be doing.
> > 
> 
> What is the use case for not caching the connection by default?

It is a matter of figuring out an invalidation strategy, so we don't
keep the other daemons running continuously when no client is actually
triggering their usage.

> virSetConnect* has very few users, but virGetConnect* is sprinkled in
> many places, including multiple uses in generic code in domain_conf.c.
> Can this be done as part of virGetConnect*?


Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|