Does a pNFS mount have the full POSIX semantics like a real local filesystem? Can you safely deliver mail on it? I've tested storage from vendors like Ibrix and Isilon and I've always found that they fail in simple scenarios, such as two clients that open the same file in O_APPEND mode. On local unix filesystems this works fine, and you get coherent results, but on most commercial cluster storage you get gibberish. The only place where I've successfully exercised the full POSIX feature set is Lustre, which works perfectly in my experience. I hope that pNFS takes after Lustre more than it takes after NFSv4.