Only noticed this message now, sorry for the delayed response!
I’ve got no direct experience using RBD with Docker Volumes, but I can relay that in Kubernetes, there’s a massive performance improvement to be gained by using RBD instead of CephFS. We switched the ElfHosted Plex volumes to RBD for this reason - it sucks a bit to loose the RWX nature of the volume, but performance for millions of little files / SQLite is much better 