The RPC-based “NFS” remote file system is sometimes considered to have slower-than-expected write performance. In NFS, a server’s RPC reply to a client write request means that the data are physically written to the server’s disk, not just placed in a queue.

(a) Explain the bottleneck we might expect, even with infinite bandwidth, if the client sends all its write requests through a single logical channel, and explain why using a pool of channels could help. Hint: You will need to know a little about disk controllers.

(b) Suppose the server’s reply means only that the data have been placed in the disk queue. Explain how this could lead to data loss that would not occur with a local disk. Note that a system crash immediately after data were enqueued does not count, because that would cause data loss on a local disk as well.

(c) An alternative would be for the server to respond immediately to acknowledge the write request and to send its own separate request later to confirm the physical write. Propose different RPC semantics to achieve the same effect but with a single logical request and reply.