[Openstack] Cells use cases

Joshua Harlow harlowja at yahoo-inc.com
Thu Oct 3 18:31:54 UTC 2013

Hi Tim,

I'd also like to know what happens above 1000 hypervisors that u think needs cells?

>From experience at y! we actually start to see the nova-scheduler (and the filter scheduler mainly) be the problem (at around ~400 hypervisors) and that seems addressable without cells (yes it requires some smart/fast coding that the current scheduler is not designed for, but that seems manageable and achievable) via reviews like https://review.openstack.org/#/c/46588, https://review.openstack.org/#/c/45867 (and others that are popping up). The filter scheduler appears to scale linearly with the number of hypervisors, and this is problematic since the filter-scheduler is also single-CPU bound (due to eventlet) so that overall, makes for some nice suckage. We haven't seen the RPC layer be a problem at our current scale, but maybe u guys have hit this. The other issue that starts to happen around ~400 is the nova service group code, which is not exactly performant when using the DB backend (we haven't tried the ZK backend yet, WIP!) due to frequent and repeated DB calls.

It'd be interesting to hear the kind of limitations u guys hit that cells resolved, instead of just fixing the underlying code itself to scale better.


From: Subbu Allamaraju <subbu at subbu.org<mailto:subbu at subbu.org>>
Date: Thursday, October 3, 2013 10:23 AM
To: Tim Bell <Tim.Bell at cern.ch<mailto:Tim.Bell at cern.ch>>
Cc: "openstack at lists.openstack.org<mailto:openstack at lists.openstack.org>" <openstack at lists.openstack.org<mailto:openstack at lists.openstack.org>>
Subject: Re: [Openstack] Cells use cases

Hi Tim,

Can you comment on scalability more? Are you referring to just the RPC layer in the control plane?


On Oct 3, 2013, at 8:53 AM, Tim Bell <Tim.Bell at cern.ch<mailto:Tim.Bell at cern.ch>> wrote:

At CERN, we’re running cells for scalability. When you go over 1000 hypervisors or so, the general recommendation is to be in a cells configuration.

Cells are quite complex and the full functionality is not there yet so some parts will need to wait for Havana.


From: Dmitry Ukov [mailto:dukov at mirantis.com]
Sent: 03 October 2013 16:38
To: openstack at lists.openstack.org<mailto:openstack at lists.openstack.org>
Subject: [Openstack] Cells use cases

Hello all,
I've really interested in  cells but unfortunately i can't find any useful use cases of them.
For instance I have 4 DCs and I need single entry point for them. In this case cells are  a bit complicated  solution. It's better to use multiple regions in keystone instead

The only one good reason for cells, which I've found, is to organize so-called failure domains, i.e. scheduling on another DCs in case of failures.

Does anyone have different use cases or vision on cells usage?
Thanks in advance.

Kind regards
Dmitry Ukov
IT Engineer
Mirantis, Inc.

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