Performance
Some of the novelties of iSCSI are related to the nature of the IP protocol. While the FC protocol is optimized for networks dedicated to connecting servers to arrays, IP-based iSCSI SANs may have to compete with nonstorage IP traffic. To minimize the impact of disruptive IP traffic, data center managers are isolating iSCSI traffic from nonstorage traffic via dedicated iSCSI networks that have no physical connection to the rest of the network, or leveraging Ethernet isolation techniques like access control lists and Virtual LANs (VLANs). "To avoid interferences from the in-house LAN, we decided to run our iSCSI network on a standalone Foundry Networks Inc. FastIron 48-port Ethernet switch," reports Kevin Mount, senior network administrator at Spokane Public Schools in Washington.

Although physical and virtual segregation greatly enhance security and performance, storage managers may need to resort to advanced Ethernet techniques like Ethernet jumbo frames and enabling flow control on the network switch and adapters to alleviate congestion and optimize throughput. In cases where the bandwidth of a single gigabit link doesn't suffice, multiple links may have to be combined...

On the host-side, TCP offload engines (TOEs) and iSCSI HBAs can save valuable CPU cycles, especially on slower or performance-critical application servers. Although TCP and iSCSI overhead of 1Gb/sec connections is less than 10% on state-of-the art server hardware and 85% of today's iSCSI deployments just use software iSCSI initiators, according to David Dale, chairman of the SNIA IP storage forum, TOEs and iSCSI HBAs will play a much larger role once 10Gb iSCSI becomes more prevalent. Besides the I/O performance boost, iSCSI HBAs come with added services like the ability to boot from a SAN and encryption.

In mixed-protocol environments, storage managers need to be aware of Ethernet oddities like erroneous speed/mode autonegotiations between Ethernet switches and network interface cards (NICs) because they can have a detrimental impact on iSCSI network performance. "To eliminate the possibility of autonegotiation problems, we hardcode all switch and server Ethernet ports," says Mount.

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