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[Openstack] [nova][cinder] Migrate instances between regions or between clusters?


Hi,

So here's a possibly stupid question - or rather, a series of such :)
Let's say a company has two (or five, or a hundred) datacenters in
geographically different locations and wants to deploy OpenStack in both.
What would be a deployment scenario that would allow relatively easy
migration (cold, not live) of instances from one datacenter to another?

My understanding is that for servers located far away from one another
regions would be a better metaphor than availability zones, if only
because it would be faster for the various storage, compute, etc.
services to communicate with each other for the common case of doing
actions within the same datacenter.  Is this understanding wrong - is it
considered all right for groups of servers located in far away places to
be treated as different availability zones in the same cluster?

If the groups of servers are put in different regions, though, this
brings me to the real question: how can an instance be migrated across
regions?  Note that the instance will almost certainly have some
shared-storage volume attached, and assume (not quite the common case,
but still) that the underlying shared storage technology can be taught
about another storage cluster in another location and can transfer
volumes and snapshots to remote clusters.  From what I've found, there
are three basic ways:

- do it pretty much by hand: create snapshots of the volumes used in
  the underlying storage system, transfer them to the other storage
  cluster, then tell the Cinder volume driver to manage them, and spawn
  an instance with the newly-managed newly-transferred volumes

- use Cinder to backup the volumes from one region, then restore them to
  the other; if this is combined with a storage-specific Cinder backup
  driver that knows that "backing up" is "creating a snapshot" and
  "restoring to the other region" is "transferring that snapshot to the
  remote storage cluster", it seems to be the easiest way forward (once
  the Cinder backup driver has been written)

- use Nova's "server image create" command, transfer the resulting
  Glance image somehow (possibly by downloading it from the Glance
  storage in one region and simulateneously uploading it to the Glance
  instance in the other), then spawn an instance off that image
  
The "server image create" approach seems to be the simplest one,
although it is a bit hard to imagine how it would work without
transferring data unnecessarily (the online articles I've seen
advocating it seem to imply that a Nova instance in a region cannot be
spawned off a Glance image in another region, so there will need to be
at least one set of "download the image and upload it to the other
side", even if the volume-to-image and image-to-volume transfers are
instantaneous, e.g. using glance-cinderclient).  However, when I tried
it with a Nova instance backed by a StorPool volume (no ephemeral image
at all), the Glance image was zero bytes in length and only its metadata
contained some information about a volume snapshot created at that
point, so this seems once again to go back to options 1 and 2 for the
different ways to transfer a Cinder volume or snapshot to the other
region.  Or have I missed something, is there a way to get the "server
image create / image download / image create" route to handle volumes
attached to the instance?

So... have I missed something else, too, or are these the options for
transferring a Nova instance between two distant locations?

Thanks for reading this far, and thanks in advance for your help!

Best regards,
Peter

-- 
Peter Penchev  openstack-dev at storpool.com  https://storpool.com/
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