On Wed, 24 Oct 2018 10:54:31 +1100, Sam Morrison wrote:
Hi nova devs,

Have been having a good look into cellsv2 and how we migrate to them (we’re still on cellsv1 and about to upgrade to queens and still run cells v1 for now).

One of the problems I have is that now all our nova cell database servers need to respond to API requests. With cellsv1 our architecture was to have a big powerful DB cluster (3 physical servers) at the API level to handle the API cell and then a smallish non HA DB server (usually just a VM) for each of the compute cells.

This architecture won’t work with cells V2 and we’ll now need to have a lot of highly available and responsive DB servers for all the cells.

It will also mean that our nova-apis which reside in Melbourne, Australia will now need to talk to database servers in Auckland, New Zealand.

The biggest issue we have is when a cell is down. We sometimes have cells go down for an hour or so planned or unplanned and with cellsv1 this does not affect other cells. Looks like some good work going on here https://review.openstack.org/#/q/status:open+project:openstack/nova+branch:master+topic:bp/handling-down-cell

But what about quota? If a cell goes down then it would seem that a user all of a sudden would regain some quota from the instances that are in the down cell?
Just wondering if anyone has thought about this?

Yes, we've discussed it quite a bit. The current plan is to offer a policy-driven behavior as part of the "down" cell handling which will control whether nova will:

a) Reject a server create request if the user owns instances in "down" cells

b) Go ahead and count quota usage "as-is" if the user owns instances in "down" cells and allow quota limit to be potentially exceeded

We would like to know if you think this plan will work for you.

Further down the road, if we're able to come to an agreement on a consumer type/owner or partitioning concept in placement (to be certain we are counting usage our instance of nova owns, as placement is a shared service), we could count quota usage from placement instead of querying cells.


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