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[cinder] Ceph, active-active and no coordination


Dear Cinder Masters,

I have a question for you. (or two, or several; well, actually the
whole Kolla team has :-) )

The background is that Kolla has been happily deploying multinode
cinder-volume with Ceph RBD backend, with no coordination configured,
cluster parameter unset, host properly set per host and backend_host
normalised (as well as any other relevant config) between the
cinder-volume hosts.

The first question is: do we correctly understand that this was an
active-active deployment? Or really something else?

Now, there have been no reports that it misbehaved for anyone. It
certainly has not for any Kolla core. The fact is it was brought to
our attention because due to the drop of Kolla-deployed Ceph, the
recommendation to set backend_host was not present and users tripped
over non-uniform backend_host. And this is expected, of course.

The second and final question is, building up on the first one, were
we doing it wrong all the time?
(plus extras: Why did it work? Were there any quirks? What should we do?)

PS: Please let me know if this thought process is actually
Ceph-independent as well.

-yoctozepto