Re: [PATCH 0/3] migration: Add COLO multifd support and COLO migration unit test

[Peter Xu <peterx@redhat.com>]

On Sun, Jan 04, 2026 at 01:44:52PM +0800, Zhang Chen wrote:
> On Tue, Dec 30, 2025 at 11:02 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Tue, Dec 30, 2025 at 03:05:43PM +0100, Lukas Straub wrote:
> > > Hello everyone,
> > > This adds COLO multifd support and migration unit tests for COLO migration
> > > and failover.
> >
> > Hi, Lukas,
> >
> > I'll review the series after the new year.
> >
> > Could you still introduce some background on how you're deploying COLO?  Do
> > you use it in production, or for fun?
> >
> > COLO is still a nice and interesting feature, said that, COLO has quite a
> > lot of code plugged into migration core.  I wished it's like a multifd
> > compressor which was much more self-contained, but it's not.  I wished we
> > can simplify the code in QEMU migration.
> >
> > We've talked it through before with current COLO maintainers, it looks to
> > me there aren't really much users using it in production, meanwhile COLO
> > doesn't look like a feature to benefit individual QEMU users either.
> >
> > I want to study the use case of COLO in status quo, and evaluate how much
> > effort we should put on it in the future.  Note that if it's for fun we can
> > always use a stable branch which will be there forever.  We'll need to
> > think about QEMU evolving in the future, and what's best for QEMU.
> >
> > Thanks,
> >
> 
> Hi Lukas and Peter,

Hi, Chen,

> 
> Thanks for this series, I will support for background info if Peter
> have any questions.

Thanks, I believe my major question so far was, whether we should deprecate
COLO in migration framework. :)

The netfilters and rest can be discussed separately.

Now looking back at my initial ask in Zhijian's fix, I still agree with
Zhijian on these two points mentioned:

https://lore.kernel.org/all/b2eadde7-57e9-426c-8487-e500ba06410e@fujitsu.com/

That is:

        - Active users who depend on it.
        - A unit test for the COLO framework.

Meanwhile, I can't see how COLO would win if to be compared with some
app-level HA infrastructure.. considering the overhead it requires on
running two VMs and compare every packet.

Lukas, thanks for trying to fix the 2nd.  I apologize that I still
requested you to send these patches, without further raising the attention
that I still want to discuss deprecation.  I don't think anyone yet proved
we should keep COLO.  I do plan to send one patch adding COLO framework to
deprecation, if nobody would stop me in a week justifying question 1 above.

We kind of proved almost nobody is actively using COLO anymore in the past
few releases.  If nobody is using COLO, we should simply drop it.

> And CC Hailiang Zhang, although he hasn't replied to emails for a long time.
> If no one objects, I think Lukas can replease Hailiang for COLO Framework.
> 
> COLO Framework
> M: Hailiang Zhang <zhanghailiang@xfusion.com>
> S: Maintained
> F: migration/colo*
> F: include/migration/colo.h
> F: include/migration/failover.h
> F: docs/COLO-FT.txt

Right, this is also another reason why I think we may want to deprecate
COLO framework.

Since I requested this series (sorry again, Lukas), I'll review it today no
matter if we decide to merge this series at last, or deprecate COLO
framework.

Thanks,

> 
> Thanks
> Chen
> 
> > >
> > > Regards,
> > > Lukas
> > >
> > > Signed-off-by: Lukas Straub <lukasstraub2@web.de>
> > > ---
> > > Lukas Straub (3):
> > >       multifd: Add colo support
> > >       migration-test: Add -snapshot option for COLO
> > >       migration-test: Add COLO migration unit test
> > >
> > >  migration/meson.build              |   2 +-
> > >  migration/multifd-colo.c           |  57 ++++++++++++++++++
> > >  migration/multifd-colo.h           |  26 +++++++++
> > >  migration/multifd.c                |  14 ++++-
> > >  tests/qtest/meson.build            |   7 ++-
> > >  tests/qtest/migration-test.c       |   1 +
> > >  tests/qtest/migration/colo-tests.c | 115 +++++++++++++++++++++++++++++++++++++
> > >  tests/qtest/migration/framework.c  |  69 +++++++++++++++++++++-
> > >  tests/qtest/migration/framework.h  |  10 ++++
> > >  9 files changed, 294 insertions(+), 7 deletions(-)
> > > ---
> > > base-commit: 942b0d378a1de9649085ad6db5306d5b8cef3591
> > > change-id: 20251230-colo_unit_test_multifd-8bf58dcebd46
> > >
> > > Best regards,
> > > --
> > > Lukas Straub <lukasstraub2@web.de>
> > >
> >
> > --
> > Peter Xu
> >
> 

-- 
Peter Xu