Hello,

We have a medium-sized Ceph Luminous cluster that, up til now, has been the
RBD image backend solely for an OpenStack Newton cluster that's marked for
upgrade to Stein later this year.

Recently we deployed a brand new Stein cluster however, and I'm curious
whether the idea of pointing the new OpenStack cluster at the same RBD
pools for Cinder/Glance/Nova as the Luminous cluster would be considered
bad practice, or even potentially dangerous.

One argument for doing it may be that multiple CInder/Glance/Nova pools
serving disparate groups of clients would come at a PG cost to the cluster,
though the separation of multiple, distinct pools also has its advantages.
The UUIDs generated for RBD images in the pools by OpenStack services
*should* be unique and collision-less between the 2 OpenStack clusters, in
theory.

One other point I was curious about was RBD image feature sets; Stein Ceph
clients will be running later versions of Ceph libraries than Newton
clients. If the 2 sets of clients were to share pools, would that itself
cause problems (in the case that neither set needed to share RBD images
within pools, only the pool itself) with some images in the pool having
different feature lists?

-- 
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Paul Browne
Research Computing Platforms
University Information Services
Roger Needham Building
JJ Thompson Avenue
University of Cambridge
Cambridge
United Kingdom
E-Mail: pf...@cam.ac.uk
Tel: 0044-1223-746548
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