Open Stack

On 10/12/2016 06:10 AM, Giulio Fidente wrote:
> hi,
>
> we introduced support for the deployment of Ceph in the liberty release
> so that it could optionally be used as backend for one or more of
> Cinder, Glance, Nova and more recently Gnocchi.
>
> We used to deploy Ceph MONs on the controller nodes and Ceph OSDs on
> dedicated ceph-storage nodes so a deployment of OpenStack with Ceph
> would need at least 1 more additional node to host a Ceph OSD.
>
> In our HA scenario the storage backends are configured as follows:
>
> Glance -> Swift
> Nova (ephemeral) -> Local
> Cinder (persistent) -> LVM (on controllers)
> Gnocchi -> Swift
>
> The downside of the above configuration is that Cinder volumes can not
> be replicated across the controller nodes and become unavailable if a
> controller fails, while production environments generally expect
> persistent storage to be highly available. Cinder volumes instead could
> even get lost completely in case of a permanent failure of a controller.
>
> With the Newton release and the composable roles we can now deploy Ceph
> OSDs on the compute nodes, removing the requirement we had for an
> additional node to host a Ceph OSD.
>
> I would like to ask for some feedback on the possibility of deploying
> Ceph by default in the HA scenario and use it as backend for Cinder.

+1 from me.  It sounds like our current default is inappropriate for an 
HA environment anyway, so if someone is using it they're already broken 
by design.  Hopefully everyone is already setting up Ceph or some other 
shared storage backend in HA so changing the default should be largely a 
non-event.  Obviously we would still need to provide an upgrade path for 
anyone who did deploy the old default (maybe they don't use Cinder and 
don't care if it's HA, for example).

>
> Also using Swift as backend for Glance and Gnocchi is enough to cover
> the availability issue for the data, but it also means we're storing
> that data on the controller nodes which might or might not be wanted; I
> don't see a strong reason for defaulting them to Ceph, but it might make
> more sense when Ceph is available; feedback about this would be
> appreciated as well.
>
> Finally a shared backend (Ceph) for Nova would allow live migrations but
> probably decrease performances for the guests in general; so I'd be
> against defaulting Nova to Ceph. Feedback?

Agreed.  It's simple enough for people to set Nova to use Ceph if they 
want, but if people haven't spec'd their compute nodes to handle heavy 
converged Ceph usage I suspect performance would be unacceptable for VMs.