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On 08/30/2017 09:09 AM, Matt Riedemann 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].

I completely agree with all of this.

Now that nova properly tracks non-shareable resources over cold migration 
(things like hugepages and PCI devices that cannot be shared) it really doesn't 
make sense to bypass the scheduler since it could end up seriously confusing the 
resource tracking mechanisms.

(We might even want to fail a live migration/evacuation with a forced 
destination that could cause a conflict in these non-shareable resources, but 
that'd be a behaviour change and therefore a new microversion.)

Chris