> Since mmap I/O is popular, together with long lived processes like databases, snapshot on open/close might not help you much.
Classic versioned file systems were generally not designed for the use case of databases, they were designed for the use case of lots of small text files (config files, source code, shell scripts, etc). For something like a database file you would turn versioning off
With these caveats, using FUSE it should be easy to do, wonder why noone actually implemented it.