[openstack-dev] [nova] Supporting force live-migrate and force evacuate with nested allocations

Matt Riedemann mriedemos at gmail.com
Wed Oct 10 14:14:11 UTC 2018

On 10/10/2018 7:46 AM, Jay Pipes wrote:
>> 2) in the old microversions change the blind allocation copy to gather
>> every resource from a nested source RPs too and try to allocate that
>> from the destination root RP. In nested allocation cases putting this
>> allocation to placement will fail and nova will fail the migration /
>> evacuation. However it will succeed if the server does not need nested
>> allocation neither on the source nor on the destination host (a.k.a the
>> legacy case). Or if the server has nested allocation on the source host
>> but does not need nested allocation on the destination host (for
>> example the dest host does not have nested RP tree yet).
> I disagree on this. I'd rather just do a simple check for >1 provider in 
> the allocations on the source and if True, fail hard.
> The reverse (going from a non-nested source to a nested destination) 
> will hard fail anyway on the destination because the POST /allocations 
> won't work due to capacity exceeded (or failure to have any inventory at 
> all for certain resource classes on the destination's root compute node).

I agree with Jay here. If we know the source has allocations on >1 
provider, just fail fast, why even walk the tree and try to claim those 
against the destination - the nested providers aren't going to be the 
same UUIDs on the destination, *and* trying to squash all of the source 
nested allocations into the single destination root provider and hope it 
works is super hacky and I don't think we should attempt that. Just fail 
if being forced and nested allocations exist on the source.




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