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Hi, Jay:

	Thanks for your detailed description. It's very helpful for me.


----------
zhangleiqiang (Trump)

Best Regards


> -----Original Message-----
> From: Jay Pipes [mailto:jaypipes at gmail.com]
> Sent: Sunday, March 30, 2014 6:02 PM
> To: openstack at lists.openstack.org
> Subject: Re: [Openstack] [nova][cinder] Question about Nova cells and Global
> Cinder
> 
> On Fri, 2014-03-28 at 07:35 +0800, zhangleiqiang at gmail.com wrote:
> > Hi, stackers:
> >
> > I am confused about reading the original BP ([1]) and operation guide
> > of nova-cells, the question as follows:
> >
> > One aim of nova-cells is to allow additional scaling and (geographic)
> > distribution without complicated database or message queue clustering,
> 
> I'm not entirely sure I agree with the geographic distribution part, but yes, the
> idea of Nova cells is to partition a large (>220-250 compute nodes, not VMs)
> deployment of Nova into sub-deployments (cells) that have their own database
> and message bus.
> 
> >  and OpenStack Compute cells are designed to allow running the cloud
> > in a distributed fashion without having to use more complicated
> > technologies, or being invasive to existing nova installations.
> 
> Hmm, well, distributed is kind of a misgnomer here. All of OpenStack is
> distributed. The main idea of cells is to scale beyond the theoretical limitations
> of a "typical" database and message queue cluster, and yes, Nova cells enables
> you to grow a Nova deployment without buying expensive scale-up hardware to
> manage increased demand on underlying database and message queue
> resources.
> 
> > The typical use case for cells is a cloud with two independent sites
> > with a single API endpoint.
> 
> IMO, that is actually not the typical use case for cells. The typical use case (is
> there one?) is to grow a *single* site's number of compute nodes greater than
> the (estimated) scaling capacity of a typical database and message queue
> cluster. "typical" and "estimated" are just
> that: one deployer's notions of what is a standard database and message
> queue setup and that deployer's experience attempting to push that database
> and MQ setup to its limits.
> 
> > However, take cinder into account, it will require Cinder act as a
> > shared service for the two sites?
> 
> I don't think there is anything in the Cinder or Nova code that prevents a Cinder
> service from being shared between two Nova cells -- or even Nova availability
> zones (which is, IMO, what you are describing here as "sites"). This all would
> depend on what storage backend you are using in Cinder, the amount of
> volume space that backend provides and can scale to, as well as the contract
> you are giving your tenants.
> 
> This last point is important. If you allow a tenant to detach a volume from a VM
> in one "site" and attach it to a VM in another site, then you will need to either
> have the Cinder database and volume backend shared between the sites or you
> will need to have some replication technology that would enable that.
> 
> Alternately, you might decide that you are *not* allowing a tenant to do that,
> but instead you will allow the tenant to *snapshot* the volume and launch a
> VM from that snapshot in a different site. This would require you to either
> share the Glance registry database between those sites or replicate that
> registry information from one site to another.
> 
> > For two independent sites with existing cloud installations, in order
> > to *upgrade* to cells later, we must make sure the storage networks of
> > these sites connected to each other as early as building the two
> > sites?
> 
> I don't know the answer to this question, unfortunately. I don't *think* this is
> necessary to do from the beginning, but sure, it would be helpful.
> 
> > In order to provide shared service for two sites, we must use the
> > storage with geographic distribution? Among current Cinder backend
> > drivers, it seems there are only glusterfs and ceph?
> 
> Or provide some replication technology external to Nova, yes.
> 
> > Furthermore, I think Cinder need to  provide the "site-affinity“
> > filter for this scenario, does it?
> 
> Yes, you would enable that filter for Cinder, but I am not entirely sure if Cinder
> understands the concept of Nova's cells, or whether the Nova cells code
> "fudges" the "site" to be the availability zone plus some cell identifier...
> 
> Best,
> -jay
> 
> 
> 
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