Comment by cryptonector

Since crashes and power failures are out of scope for POSIX, even F_FULLSYNC's behavior description would of necessity be informative rather than normative.

But, the reality is that all operating systems provide some way to make writes to persistent storage complete, and to wait for them. All of them. It doesn't matter what POSIX says, or that it leaves crashes and power failure out of scope.

POSIX's model is not a get-out-of-jail-free card for actual operating systems.