It might be possible to replace freqfs with mmap on a POSIX OS, but a) you would still have to implement your own read-write lock, and b) you would (I think probably?) lose some consistency in behavior across different host operating systems.

Which OSes does this run on that doesn’t have some kind of mmap operation?

It should work on Windows (because tokio::fs works on Windows) although I have not personally tested this

You can do mmap on Windows, eg. https://github.com/danburkert/memmap-rs

mmaps for read, explicit API for writing, a-la LMDB. Buggy readers can read inconsistent data but cannot corrupt the os.

Corrupt the OS? How might that happen?

Sorry, I meant the DB!