[openstack-dev] [nova] nova cellsv2 and DBs / down cells / quotas

Sam Morrison sorrison at gmail.com
Tue Oct 23 23:54:31 UTC 2018

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 <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?


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