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I've written a nova-manage placement heal_allocations CLI [1] which was 
a TODO from the PTG in Dublin as a step toward getting existing 
CachingScheduler users to roll off that (which is deprecated).

During the CERN cells v1 upgrade talk it was pointed out that CERN was 
able to go from placement-per-cell to centralized placement in Ocata 
because the nova-computes in each cell would automatically recreate the 
allocations in Placement in a periodic task, but that code is gone once 
you're upgraded to Pike or later.

In various other talks during the summit this week, we've talked about 
things during upgrades where, for instance, if placement is down for 
some reason during an upgrade, a user deletes an instance and the 
allocation doesn't get cleaned up from placement so it's going to 
continue counting against resource usage on that compute node even 
though the server instance in nova is gone. So this CLI could be 
expanded to help clean up situations like that, e.g. provide it a 
specific server ID and the CLI can figure out if it needs to clean 
things up in placement.

So there are plenty of things we can build into this, but the patch is 
already quite large. I expect we'll also be backporting this to stable 
branches to help operators upgrade/fix allocation issues. It already has 
several things listed in a code comment inline about things to build 
into this later.

My question is, is this good enough for a first iteration or is there 
something severely missing before we can merge this, like the automatic 
marker tracking mentioned in the code (that will probably be a 
non-trivial amount of code to add). I could really use some operator 
feedback on this to just take a look at what it already is capable of 
and if it's not going to be useful in this iteration, let me know what's 
missing and I can add that in to the patch.

[1] https://review.openstack.org/#/c/565886/

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Thanks,

Matt