Unfortunately, it's often not that simple on POSIX systems; it's quite common for disk controllers to disobey fsync, for instance, either in their drivers or in their hardware caches or both. I'm actually surprised Apple is even willing to make the guarantees above about F_FULLFSYNC; I'd read it as, at best, only applying to Apple hardware, as if a third-party controller is doing something silly they can't really do much about that.