While reading through RFC 1813 (NFSv3 RFC), I came across the following interesting NFS error:
The server initiated the request, but was not able to complete it in a timely fashion. The client should wait and then try the request with a new RPC transaction ID. For example, this error should be returned from a server that supports hierarchical storage and receives a request to process a file that has been migrated. In this case, the server should start the immigration process and respond to client with this error.
This is nifty, and leads me to wonder if any NFS-based HSM solutions are utilizing this.