The conceptual relationship to PlanetScale seems somewhat dubious to me (as mentioned in other comments) but the idea of implementing snapshots and branching below the database server level, in the storage layer using Ceph, is neat and potentially quite elegant! Kudos for that idea :-). I hope implementing it works out to the same elegant API envisioned in the initial concept. I could see this being very useful in a development workflow like the example (dev-db/feature-login).

I have to say that my experiences with running virtualized relational database servers on top of Ceph centralized storage have shown somewhat disappointing performance, so I think that could become a real challenge if performance is or becomes a goal. However, I've encountered multiple mentions that Ceph performance is supposed to increase significantly as the number of storage nodes increases (from a handful to a dozen to much larger clusters) and while I cannot corroborate this from personal experience, it stands to reason that "throwing more (hardware) resources at the problem" can make a big difference.