Re: [Xen-devel] Migration v2 and related work for 4.6

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2015-04-15 at 13:57 +0100, Andrew Cooper wrote:
> On 15/04/15 13:52, Ian Campbell wrote:
> > On Tue, 2015-04-07 at 12:59 +0100, Wei Liu wrote:
> >> On Wed, Apr 01, 2015 at 12:03:51PM +0100, Andrew Cooper wrote:
> >>> Hello,
> >>>
> >>> I believe I have CC'd all interested parties.  If I have missed anyone,
> >>> please shout.
> >>>
> >>> There are several large series:
> >>> * Cancellable async operations
> >>> * COLO
> >>>
> >>> and a wish to start experimenting with migration on ARM which all
> >>> interact with my large series for migration v2 support in libxc and libxl.
> >>>
> >>> I am working on the libxl side of things, but realise that I am being a
> >>> blockage to the other areas waiting on migration v2.
> >>>
> >>> The current state of the libxc bit is functionally complete.  It is
> >>> shipping in two versions of XenServer, has not changed substantially in
> >>> 9 months, and not changed in a backwards incompatible way since v1.
> >>>
> >>> I propose that the libxc series be accepted independently of the libxl
> >>> series.  The libxc series is still implemented as an API-compatible
> >>> alternative to legacy migration, and is not enabled by default, meaning
> >>> no change to current users.
> >>>
> >>> However, it will unblock development of ARM migration and COLO, and will
> >>> allow people in the wider community to experiment with migration v2.  An
> >>> existing `xl migrate` can be swapped from legacy to migration v2 simply
> >>> by setting an environment variable (for PV guests.  HVM requires the
> >>> libxl changed as the handling of the Qemu save record is changing.)
> >>>
> >>> I feel that this is best interest for the community, to get some testing
> >>> available earlier in the development cycle, rather than attempting to
> >>> splice 3 large series in a related area together towards the code freeze.
> >>>
> >>> Thoughts? (especially from a release management point of view)
> >>>
> >> I've looked at your libxc series. It looks complete. I agree it would be
> >> good to get it in as early as possible.
> > I think the question is more at what point should we flip the switch and
> > make that new code the default without needing to set magic envvars?
> >
> > Or to take it a step further, when can we delete all the old code.
> 
> The other bits needed are:
> 
> * Remus/Migrationv2 (not too much, judging from the rfc set)
> * A decision regarding tmem.  That is one bit not covered at all any of
> the new code, or the legacy conversion.

Adding the tmem maintainer.

Konrad, I think you tmem guys need to step up to the plate and make tmem
work with migration v2, AIUI this has been known for a while and I'm not
in favour of stalling migr2 while we wait.

> * Libxl/migrationv2

> The libxl migration v2 will contain the legacy->v2 conversion scripts,
> which will cover the backwards compatibility aspect.

Ah, OK, and this is why we can't just throw the switch on the libxc side
and do libxl later (for 4.6 or not). I think you've explained this to me
repeatedly and I keep forgetting...

> It is my hope that I can submit a very large pruning patch in time for 4.6.

Great!

Ian.


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