i would vote for instead of using glance urls  to make a first version that puts it directly into swift with a configurable ttl  defaulting to a few days  in the message to ironic  instead of using a glance url  pass a swift temporary url to ironic then you do not need to tackle all of the complexities of garbage collection in glance just let swift manage it with a ttl  in the long run the storage would need to be plug able for different deployments  but swift seems like a reasonable first stab 