RBD clones without snapshots

[danielnachtrub]

The current CSI implementation supports cloning of images. Doing clones always takes snapshots. This is fine in most cases except we're trying to clone a mirrored image on the secondary site - snapshots are not supported on mirrored images.

In our use case we want to mirror images and freeze them in time (the best way for now is to use clones as snapshots are not supported by ceph itself) to have long term retention. Cloning with rbd-default-clone-format 2 works because ceph doesn't require a snapshot of the image to issue the clone.

While we can orchestrate this on our own, it might make more sense to use the CSI to issue the clones. Having the snapshots hardcoded in the clone path while ceph supports (since mimic) the snapshotless clone i'm wondering if there is a specific reason to do so.

Would it make sense to review this behavior and add support for snapshotless clones? We can contribute a PR for this - it would just be nice to get into discussion before to see if it's worth the effort or if there would be other reasons it doesn't make any sense.