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The advantage using NFS over TCP is that it works far better than UDP over unstable networks. When using TCP, a single dropped packet can be retransmitted, without the retransmission of the entire RPC request, resulting in better performance over unstable networks.
In addition, TCP will handle network speed differences better than UDP, due to the underlying flow control at the network level. When using UDP, if network speeds of the client and server are not identical, dropped packets and retransmissions might cause performance to be extremely slow when the faster entity tries to send data to the slower entity.
The overhead incurred by the TCP protocol will result in somewhat slower performance than UDP under ideal network conditions, but the cost is not severe, and is often not noticeable without careful measurement.
By default, the client will attempt to mount NFS export with TCP if supported, you can also use option proto=tcp to explicitly use TCP.
Note: NFS over TCP is integrated into all Linux kernel 2.4 and later.