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Re: Share Glance between cells or regions


On 05/15/2013 02:46 PM, John Paul Walters wrote:
> Hi,
> We're looking at setting up a geographically distributed OpenStack installation, and we're considering either cells or regions.  We'd like to share a single Glance install between our regions (or cells), so the same images can be spawned anywhere.  From here:
> http://docs.openstack.org/trunk/openstack-ops/content/segregate_cloud.html
> it's not clear whether that's possible.  Can anyone shed some light on this?  Is it possible in regions OR cells (or both)?  Is there a better solution that I'm not thinking of?  

We will be sharing both the Keystone identity (note: not token/catalog)
and Glance registry databases in a synchronously-replicated Galera MySQL
cluster. Databases like the above, which have extremely low write to
read ratios are ideal for this kind of replication. We are replicating
working sets over the WAN using rsync replication in the WSREP
clustering software.

What this enables us to do is have a single set of account records and a
single set of image (base and snapshot) records. Note that we back
Glance in each zone with a zone-local Swift cluster. But what this
allows us to do is have a user in zone A make a snapshot and then
immediately (once the snapshot goes from the SAVING state to ACTIVE),
the user is able to launch their snapshot in zone B. The Glance registry
database has the location of the snapshot in zone A's Swift cluster and
when Nova in zone B launches the image, the Glance API server in zone B
simply pulls the image bits from Swift in zone A.


p.s. I say "will be sharing" because we are currently updating our
deployment to use this single Glance registry database. Originally we
went down the route of each zone having its own Glance registry database
and realized that since the pattern of write activity to the Glance
registry is so low, it made sense to replicate it across our zones and
give the users the ability to launch instances from snapshots in any
zone. The single identity database is already in use across our zones.

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