Today, storage area networks (SAN) are widely used in data center networks to connect servers to data storage devices. In such a network, ensuring zero packet loss is very important for a server's network file system (NFS) to quickly access data across the network. Otherwise, the time spent on detecting packet losses and then retransmitting the lost data will severely harm the data access throughput and delay performance of the network file system. Recently, the SDN technology has been widely used in data center networks. In an SDN network, the SDN controller has a global view of the states of the overall network. Exploiting this capability, we designed SDN-assisted rate-based and credit-based congestion control schemes to ensure zero packet loss in the network. We used both greedy traffic and bursty NFS traffic to compare the performances of the two schemes under many different situations. In this paper, we report their relative strength and weakness and show that the SDN-assisted credit-based scheme can ensure zero packet loss while maintaining a high link utilization.