[libvirt] [PATCH v2 5/5] qemu: Enable memory-backend-file.discard-data whenever possible

[Daniel P. Berrangé]

On Tue, Apr 17, 2018 at 02:57:50PM +0200, Peter Krempa wrote:
> On Tue, Apr 17, 2018 at 13:18:02 +0100, Daniel Berrange wrote:
> > On Tue, Apr 17, 2018 at 01:16:42PM +0200, Michal Privoznik wrote:
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1480668
> > > 
> > > The cases when we cannot enable this optimization are:
> > >   1) nvdimms
> > >   2) if memAccess='shared'
> > > 
> > > Otherwise it is safe to enable it.
> > 
> > This is basically saying that if memAccess==private, then no person/app cares
> > about the contents of the memory-backend-file storage after QEMU has exited,
> > because (implicitly) whatever data was in that storage is not going to be used
> > again. While I accept that's going to be a common case, I'm wary of saying that
> > is a 100% safe/valid assumption for every user.
> > 
> > I tend think it should require an explicit opt-in to turn on this behaviour
> 
> Well, we used that behaviour prior to storing the backing files in a
> "predictible" path as the file was unlinked right after creating it
> (so that some memory management software can do some magic behind our
> backs with that) so I don't think anybody would depend on the described
> behaviour.

Hmm, I guess that's true.

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 :|