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Re: [Qemu-devel] [libvirt] [PULL 25/26] block: Remove deprecated -drive

From: Kevin Wolf
Subject: Re: [Qemu-devel] [libvirt] [PULL 25/26] block: Remove deprecated -drive option serial
Date: Wed, 4 Jul 2018 15:34:40 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

Am 04.07.2018 um 15:02 hat Cornelia Huck geschrieben:
> On Tue, 3 Jul 2018 13:32:29 +0200
> Kevin Wolf <address@hidden> wrote:
> > > > > Has serial/gemoetry been fixed meanwhile and will it make it into the
> > > > > next release?  
> > > > 
> > > > I cannot find an archive that has it, but it is on the libvirt mailing
> > > > list as "[libvirt] [PATCH v3] qemu: format serial and geometry on 
> > > > frontend disk device".
> > > > Review seems done, but it has missed libvirt 4.5 which was released 
> > > > today.  
> > > 
> > > Just posted latest version here:
> > > 
> > >   https://www.redhat.com/archives/libvir-list/2018-July/msg00130.html
> > > 
> > > It will be in the next release on ~ Aug 1st  
> > 
> > It would have been a lot nicer to have it the July release because this
> > means that we'll have the released libvirt broken during almost the
> > whole rc phase of QEMU 3.0, but the release is planned for Aug 8th the
> > earliest, so I guess we're still okay. People using QEMU from git will
> > just need libvirt from git as well.
> Speaking as an innocent* bystander:
> I would usually presume that I can use any recent libvirt to test
> current QEMU, even bleeding edge. In this case, not even the latest
> released libvirt version will be fine, I would also need to build
> libvirt from git (which is probably not something a non-libvirt
> developer will usually do). If everything goes according to plan, I can
> only test QEMU with a released libvirt version at the very tail end of
> hardfreeze, where only release blockers are appropriate.

I understand where you're coming from, but let's be honest: It's not as
if disk geometry or serial numbers were features that absolutely need
to be there to give QEMU any testing.

Also, my understanding has always been that we expect users to have a
libvirt version that isn't older than QEMU. It would be useful to set a
clear policy for this and document it.

> I think it would be really beneficial to general QEMU test coverage to
> push deleting this option back a release or two. We should make testing
> QEMU in conjunction with libvirt as uncomplicated as possible.

Essentially, what is important to me isn't getting these options dropped
exactly in 3.0, but not setting a bad precedence that deprecation isn't
actually worth anything. We may easily end up with this deprecation

depreate a feature
release QEMU version n + 1
release QEMU version n + 2
remove the feature
while libvirt hasn't removed use of the feature:
    # ...and why should it when everything is still working?
    reinstate the feature
    release QEMU version n + x
    remove the feature

When management tools know that this is the process, the motivation to
remove the use of the feature gets even lower (not out of malice, but
because there will be always more important things), so this will
"optimise" itself into an endless loop and we're back to never actually
removing old cruft that impedes development.

The libvirt patch has just been merged (and I'm almost sure that this
wouldn't have happened so quickly if I had just reverted the patch right
away), so at least we know now that this specific instance of the loop
is going to terminate.

What's left is first and foremost that we need to sort out our broken
deprecation mechanism, and if that gets done, I don't mind if someone
wants to revert the patch for 3.0 as long as they also take care that it
gets back into 3.1.


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