How to ensure Ethernet can support a SAN

Storage Area Networks cannot perform unless the network they rely on has been optimised for their needs. Hence this story on how to design a network to support a SAN.

With iSCSI SANS now on sale for around $5,000, it should be no surprise that small businesses are adopting the technology in droves.

If it's your job to install one, you will need to identify and put in place best practices to optimize a small business' Ethernet networks to support these iSCSI SANs. You will need to choose and deploy the right type of Ethernet switches and properly configure Ethernet switch features like flow control and jumbo frames for optimal iSCSI performance.

While choosing an Ethernet switch may seem like a no-brainer, since any Ethernet switch that supports TCP/IP traffic will support iSCSI traffic, it really is a strategic decision. The right Ethernet switch can better position a customer's iSCSI SANs for future network and storage growth, and it can improve iSCSI SAN performance. Factors to consider in this choice are the storage system the customer plans to use and the SAN's growth rate.

Here's why the storage system in use is an important factor in the choice of Ethernet switch: Some storage systems, such as Dell's EqualLogic PS Series iSCSI storage systems, are stackable systems that can create one logical virtual pool, or group, of storage. To create this storage group, every member in the group needs to communicate with the others through the network. Also, every server host needs access to every member in the Dell EqualLogic storage system group.

In that scenario, if the small business you are working with expects minimal or no growth, 24- or 48-port switches with support for interswitch links (ISLs) will be fine. ISLs connect different Ethernet switches and route TCP/IP and iSCSI traffic between them. Using switches that only support ISLs, storage systems can communicate with one another, and any server can access any storage system connected to the iSCSI SAN.

If your customer expects rapid growth of its iSCSI SAN, ISL-supported switches are not the best choice, because ISLs' use of Ethernet switch ports increases as members are added to the storage group. Instead, you should use stackable Ethernet switches for fast-growing iSCSI SANs. These switches have dedicated ports for routing TCP/IP and iSCSI traffic, keeping more ports free for server and storage connections. Stackable switches are also available from some vendors with 10 Gigabit Ethernet (10 GigE) ports, which can further improve performance on an iSCSI SAN.

Another consideration in Ethernet switches is OSI layer. Layer 3 routing is preferable for switches that have dedicated ports for TCP/IP and iSCSI traffic; Layer 3-supported switches can route traffic between switches based on the server or storage system's IP address. Layer 2 Ethernet switches route network traffic only based on MAC addresses, so if the switch cannot resolve the MAC address, it needs to send the packet to a router. Depending on the size of the iSCSI SAN and the amount of network traffic, this can become a performance bottleneck, one that Layer 3 Ethernet switches avoid.

Key Ethernet switch features

Sites that have a lot of sequential traffic, such as video streaming or backup, can benefit from the correct implementation of the jumbo frame feature on Ethernet switches. Jumbo frames optimize end-to-end data transfers by putting more data in each TCP/IP packet, which minimizes transmit and receive interrupt processing on server and storage hosts. Jumbo frames are by default about 9 KB in size, so there needs to be enough data to fill the jumbo frame, and both the initiator's and target's network cards need to support jumbo frames.

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Flow control, the process of managing data transmission rates between two nodes, is also important. Most switches handle flow control in a similar manner, but you should check the flow control settings on the host's Ethernet network cards to ensure that buffer credit allocation is sufficient for each application. With correct buffer credit allocation, the card can tell the switch to slow down traffic to avoid dropped packets. Without flow control, packets can be dropped and need retransmission.

A word of caution when using both jumbo frames and flow control: Using both features can affect storage system stability, according to Jay Kramer, iStor Networks' vice president of worldwide marketing. iStor has seen some situations where the Ethernet switch did not have enough horsepower to drive jumbo frames when flow control was enabled; this affects system stability, Kramer said. Before turning on both jumbo frames and flow control, you should verify with the storage system vendor that using these features at the same time will not negatively affect the storage system.

About the author

Jerome M. Wendt is the founder and lead analyst of The Datacenter Infrastructure Group. You can find his blog posts at www.dciginc.com.

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