A bit of late reply, but I tried to stay away, as much as I could, from NFS overall, when we talk about large scales, RAC in our case.
From the manpage of mount_nfs:
Quote:
If the filesystem is mounted using forcedirectio, data is transferred directly between client and server, with no buffering on the client. If the filesystem is mounted using noforcedirectio, data is buffered on the client. forcedirectio is a performance option that is of benefit only in large sequential data transfers. The default behavior is noforcedirectio.