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On 06/18/2018 08:16 AM, Artom Lifshitz wrote:
> Hey all,
>
> For Rocky I'm trying to get live migration to work properly for
> instances that have a NUMA topology [1].
>
> A question that came up on one of patches [2] is how to handle
> resources claims on the destination, or indeed whether to handle that
> at all.

I think getting the live migration to work at all is better than having it stay 
broken, so even without resource claiming on the destination it's an improvement 
over the status quo and I think it'd be a desirable change.

However, *not* doing resource claiming means that until the migration is 
complete and the regular resource audit runs on the destination (which could be 
a minute later by default) you could end up having other instances try to use 
the same resources, causing various operations to fail.  I think we'd want to 
have a very clear notice in the release notes about the limitations if we go 
this route.

I'm a little bit worried that waiting for support in placement will result in 
"fully-functional" live migration with dedicated resources being punted out 
indefinitely.  One of the reasons why the spec[1] called for using the existing 
resource tracker was that we don't expect placement to be functional for all 
NUMA-related stuff for a while yet.

For what it's worth, I think the previous patch languished for a number of 
reasons other than the complexity of the code...the original author left, the 
coding style was a bit odd, there was an attempt to make it work even if the 
source was an earlier version, etc.  I think a fresh implementation would be 
less complicated to review.

Given the above, my personal preference would be to merge it even without 
claims, but then try to get the claims support merged as well.  (Adding claims 
support later on wouldn't change any on-the-wire messaging, it would just make 
things work more robustly.)

Chris

[1] 
https://github.com/openstack/nova-specs/blob/master/specs/rocky/approved/numa-aware-live-migration.rst