This is like saying sync() works. You can sync to durable storage at any point, yes. But you cannot do it in a semantically useful fashion. "I don't think any

This is like saying sync() works. You can sync to durable storage at any point, yes. But you cannot do it in a semantically useful fashion. "I don't think anyone is talking about situations where B is being actively written to while the rename happens." Well, you specifically are ignoring this. I'm not. Filesystems are free to generate extra syncs whenever they like. You could even have a filesystem sync every single write operation to durable storage - why not?