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

Sam Morrison sorrison at gmail.com
Wed Oct 24 23:55:15 UTC 2018

> On 24 Oct 2018, at 4:01 pm, melanie witt <melwittt at gmail.com> wrote:
> 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.

OK great, always good to know other people are thinking for you :-) , I don’t really like a or b but the idea about using placement sounds like a good one to me.

I guess our architecture is pretty unique in a way but I wonder if other people are also a little scared about the whole all DB servers need to up to serve API requests?

I’ve been thinking of some hybrid cellsv1/v2 thing where we’d still have the top level api cell DB but the API would only ever read from it. Nova-api would only write to the compute cell DBs.
Then keep the nova-cells processes just doing instance_update_at_top to keep the nova-cell-api db up to date.

We’d still have syncing issues but we have that with placement now and that is more frequent than nova-cells-v1 is for us.


> Cheers,
> -melanie
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