Correction; Linux O_DIRECT is not Windows Direct I/O
and the Parallel NFS (pNFS) client is still in tech preview; the latter includes support for Microsoft's Direct I/O, which allows for data to be read from disk to application buffers without stopping at file buffers
I've spoken to the Linux developer of the NFS client that's part of the standard distributions, including RHEL6. The support is for Linux O_DIRECT in the pNFS code line, and has nothing to do with Microsoft's Direct I/O. That is a specific Windows OS feature for Windows device drivers.
Alex McDonald, CTO Office, NetApp.