[openstack-dev] [nova] ops meetup feedback

Tim Bell Tim.Bell at cern.ch
Tue Sep 20 15:08:14 UTC 2016


On 20 Sep 2016, at 16:38, Sean Dague <sean at dague.net<mailto:sean at dague.net>> wrote:
...
There were also general questions about what scale cells should be
considered at.

ACTION: we should make sure workarounds are advertised better
ACTION: we should have some document about "when cells"?

This is a difficult question to answer because "it depends." It's akin
to asking "how many nova-api/nova-conductor processes should I run?"
Well, what hardware is being used, how much traffic do you get, is it
bursty or sustained, are instances created and left alone or are they
torn down regularly, do you prune your database, what version of rabbit
are you using, etc...

I would expect the best answer(s) to this question are going to come
from the operators themselves. What I've seen with cellsv1 is that
someone will decide for themselves that they should put no more than X
computes in a cell and that information filters out to other operators.
That provides a starting point for a new deployment to tune from.

I don't think we need "don't go larger than N nodes" kind of advice. But
we should probably know what kinds of things we expect to be hot spots.
Like mysql load, possibly indicated by system load or high level of db
conflicts. Or rabbit mq load. Or something along those lines.

Basically the things to look out for that indicate your are approaching
a scale point where cells is going to help. That also helps in defining
what kind of scaling issues cells won't help on, which need to be
addressed in other ways (such as optimizations).

-Sean


We had an ‘interesting' experience splitting a cell which I would not recommend for others.

We started off letting our cells grow to about 1000 hypervisors but following discussions in the
large deployment team, ended up aiming for 200 or so per cell. This also allowed us to make the
hardware homogeneous in a cell.

We then split the original 1000 hypervisor cell into smaller ones which was hard work to plan.

Thus, I think people who think they may need cells are better adding new cells than letting their first one
grow until they are forced to do cells at a later stage and then do a split.

Tim

--
Sean Dague
http://dague.net

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