Abstract: This paper describes a virtual file system that allows data to be transferred on demand between storage and compute servers for the duration of a computing session. The solution works with unmodified applications (even commercial ones) running on standard operating systems and hardware. The virtual file system employs software proxies to broker transactions between standard NFS clients and servers; the proxies are dynamically configured and controlled by computational grid middleware. The approach has been implemented and extensively exercised in the context of the Purdue University Network Computing Hubs, an operational computing portal that has more than 1,500 users across 24 countries. Results show that the virtual file system performs well in comparison to native NFS: performance analyses show that the proxy incurs mean overheads of 1% and 18% with respect to native NFS for a single-client execution of the Andrew benchmark in two representative computing environments, and that the average overhead for eight clients can be reduced to within 1% of native NFS with concurrent proxies.