I think its great that we're having this discussion.

In the hope that its informative, I'd like to give some info on issues we're looking at when moving our Glance deployment to Folsom. A lot of this is in common with Ryan, but there are a couple of twists due to our goals of maximization of uptime (ie we are hoping to do a rolling rather than lock-step upgrade) and decoupling upgrades. Also, I mention the case where you may have existing Glance clients which you don't control.

In our case the upgrade of the various components (swift/glance/nova etc) will be staggered rather than performed simultaneously. (For large organisations I imagine this may often be the case.) If we are to avoid downtime for Glance we must simultaneously support ids and uuids. These must be presented appropriately to Nova nodes which we must assume are moving targets -- ie will initially be on older code, but will upgrade to Folsom.

We have some ideas on how this may be possible but haven't worked through all the details yet (in particular the Nova database entries)... but there could be some coding for Nova/Glance and some deploys of the interoperable code before eventually switching to standard Folsom.

(Jay -- I don't think scripts are sufficient here?)

If Glance were publically available its not clear how the id/uuid change could be worked through gracefully where we didn't have control over the glance client. Ie the upgrade would break existing customers' clients which expected an id rather than a uuid.