Yes, NFS is too weak. POSIX is too restrictive. I'd like to point out that
especially in the supercomputing domain there is a "third way" to do
consistency semantics: the MPI standard defines "MPI-IO consistency
semantics", which give applications assurances as to when data will be
visible (no bizzaro NFS client-side caching behaviors) but relax the POSIX
rules in ways that only help performance.
Because you asked: distributed file systems that implement POSIX consistency