There's an NFSv3 COMMIT operation, combined with a "durability" marker on writes. fsync could translate to COMMIT, but if writes are marked as "durable", COMMIT is not called by common clients, and if writes are marked as non-durable, COMMIT is called after every operation, which kind of defeats the point. When you use NFS with ZeroFS, you cannot really rely on "fsync".

I'd recommend using 9P when that matters, which has proper semantics there. One property of ZeroFS is that any file you fsync actually syncs everything else too.