[Openstack-operators] [openstack-dev] [nova] Should we add the 'force' option to the cold migrate API too?

Sylvain Bauza sbauza at redhat.com
Thu Aug 31 14:51:23 UTC 2017

On Wed, Aug 30, 2017 at 5:09 PM, Matt Riedemann <mriedemos at gmail.com> wrote:

> Given the recent bugs [1][2] due to the force flag in the live migrate and
> evacuate APIs related to Placement, and some other long standing bugs about
> bypassing the scheduler [3], I don't think we should add the force option
> to the cold migrate API, as (re-)proposed in Takashi's cold migrate spec
> here [4].
> I'm fine with being able to specify a host during cold migrate/resize, but
> I think the specified host should be validated by the scheduler (and
> placement) so that the instance can actually move to that specified
> destination host.
> Since we've built more logic into the scheduler in Pike for integration
> with Placement, bypassing that gets us into maintenance issues with having
> to duplicate code throughout conductor and just in general, seems like a
> bad idea to force a host and bypass the scheduler and potentially break the
> instance. Not to mention the complicated logic of passing the host through
> from the API to conductor to the scheduler is it's own maintenance problem
> [5].
> Looking back at when the force flag was added to the APIs, it was from
> this spec [6]. Reading that, before that microversion if a host was
> specified we'd bypass the scheduler, so the force flag was really just
> there for backward compatibility

Indeed. That said, I've heard some ops wanting to migrate instances to
computes where resources were not possibly enough to accept the instance
but where it's preferred to have performance problems than just stopped
If you think about the move operations using the force flag (evacuate and
live-migrate), those were used by operators when they had a problem with a
compute node and they wanted to *evacuate* very quickly instances.

> I guess in case you wanted the option to break the instance or your
> deployment. :) Otherwise after that microversion if you specify a host but
> not the force flag, then we validate the specified host via the scheduler
> first. Given this, and the fact we don't have any backward compatibility to
> maintain with specifying a host for cold migrate, I don't think we need to
> add a force flag for it, unless people really love that option on the live
> migrate and evacuate APIs, but it just seems overly dangerous to me.

While I understand operators wanting to *evacuate* instances (or rebuilding
them by using the evacuation API) in case they see problems with hosts, I
don't see why we should need to have a "force" flag for a cold migration if
you're passing a target.
Say :
 - either your compute node is down and then you need to recreate your
customers' instances very quickly : then you call "nova evacuate".
 - or your compute node is still alive but you want to migrate quickly
without telling your customers : then you use "nova live-migrate".

I don't see cases where operators (because passing a target requires you to
be an admin)  would like to cold migrate instances for their customers
without communicating them a specific timeline for the move operation and
so quickly that it would require to use a force flag to bypass the
Maybe I'm wrong but I'm fine with asking Takashi to not add the force flag
in his implementation for the cold migration API and wait for people
wanting to have that flag to propose a specific specification that would
describe the use-case.

> Finally, if one is going to make the argument, "but this would be
> consistent with the live migrate and evacuate APIs", I can also point out
> that we don't allow you to specify a host (forced or not) during unshelve
> of a shelved offloaded instance - which is basically a move (new build on a
> new host chosen by the scheduler). I'm not advocating that we make unshelve
> more complicated though, because that's already broken in several known
> ways [7][8][9].

Well, we don't have consistent APIs anyway. If you think about all the move
operations plus the boot request itself, each of them is *already* very
different from the other from an API perspective. Yay.

> [1] https://bugs.launchpad.net/nova/+bug/1712008
> [2] https://bugs.launchpad.net/nova/+bug/1713786
> [3] https://bugs.launchpad.net/nova/+bug/1427772
> [4] https://review.openstack.org/#/c/489031/
> [5] http://lists.openstack.org/pipermail/openstack-dev/2017-Augu
> st/121342.html
> [6] https://specs.openstack.org/openstack/nova-specs/specs/mitak
> a/implemented/check-destination-on-migrations.html
> [7] https://bugs.launchpad.net/nova/+bug/1675791
> [8] https://bugs.launchpad.net/nova/+bug/1627694
> [9] https://bugs.launchpad.net/nova/+bug/1547142
> --
> Thanks,
> Matt
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