Choosing the right switch for an iSCSI SAN

ISCSI storage doesn't actually require a switch. However, some iSCSI switches offer advanced capabilities that can increase SAN performance. This article offers information about choosing an iSCSI switch and a user case study.

Eric Nelson, director of information technology at St. Joseph Healthcare, a major hospital in Bangor, Maine, didn't set out to acquire an iSCSI switch. He just needed to replace an aging Fibre Channel (FC) SAN that was becoming too costly and difficult to maintain. What he ended up with was an iSCSI switch from Sanrad Inc., and more.

ISCSI storage doesn't actually require a switch. An organization that wants iSCSI storage can use an iSCSI initiator, which is often free, and connect the server directly to the iSCSI array sitting on an Ethernet network. It's simple, cheap and easy.

Fibre Channel over Ethernet
Fibre Channel over Ethernet (FCoE) provides network convergence at the cabling level through a single Ethernet cable handling both Fibre Channel and IP. FCoE will appeal to enterprises with FC SANs that are not migrating to iSCSI and Ethernet. The appeal of FCoE is the ability to tap fast 10 Gbps Ethernet speeds for legacy FC SANs.

For the storage administrator, the storage network still looks like traditional FC, only faster. "FCoE maintains all FC addressing, zoning, and security," says Doug Ingraham, senior director or product management at Brocade. It makes Ethernet lossless by working at the hardware layer to ensure no lost frames.

Neither FCoE nor Converged Enhanced Ethernet (CEE) is here today. "FCoE has been ratified and products will begin shipping in 2009," says Ingraham. The details of CEE are still being hammered out in standards committee. 10 Gbps Ethernet is shipping but currently remains expensive but costs are expected to drop.

The issue for iSCSI storage, however, goes beyond the switches. Storage connectivity is on the cusp of major changes. The advent of 10 Gigabit Ethernet (GigE) and 8 Gb FC are leading the storage industry to rethink storage networking through such technologies as Converged Enhanced Ethernet (CEE) and FC over Ethernet (FCoE).

Cheap and easy

The iSCSI appeal is simplicity and low cost. "iSCSI lowers the cost of building storage SANs by using Ethernet NICs and switches instead of more expensive FC host bus adapters (HBAs), switches and directors," according to the latest Gartner iSCSI Marketscope.

Networking represents much of the iSCSI attraction. "Most, if not all, savings associated with iSCSI are in connectivity -- host connections and switch ports," says Roger Cox, analyst, in the Gartner iSCSI Marketscope. Compared to high-performance enterprise FC storage, iSCSI could provide tenfold savings, the Gartner report notes. That becomes especially apparent when the subject turns to iSCSI switches. "Any standard Ethernet switch will do the job," says Cox, including cheap switches you might find at a discount office superstore.

"There is no secret sauce when it comes to iSCSI and Ethernet switches," says Mike McNamara, product marketing manager at NetApp. iSCSI storage requires no particular features beyond basic switching.

"Yes, you can use any switch. However, you want to include some advanced capabilities, particularly if you are concerned about the performance of the SAN," says Lin Nease, director of emerging business for ProCurve, Hewlett-Packard's switch brand. Adds Doug Ingraham, senior director or product management at Brocade: "Basically, the better the switch you buy, the better off you'll be."

The extra goodies you might be interested in address reliability and network control. For example, switches that support jumbo frames, offer VLAN capabilities, or deliver capabilities to support quality of service (QoS) will be useful but not essential for iSCSI storage.

As for cost, iSCSI switches remain a bargain. Depending on the speed and built-in features, FC switches run almost $1,000 per port, often more, according to published prices. Ethernet switches used for iSCSI SANs, by comparison, typically cost $400 to $600 per port, or less.

Converged Enhanced Ethernet

The problem with Ethernet for iSCSI storage is that "storage is not tolerant of dropped packets," says Deepak Munjal, senior marketing manager, Cisco Systems Inc. Ethernet drops packets that must be resent, which slows down the performance of iSCSI over the network. "You want a switch capable of handling dropped packets," says Munjal.

CEE, called Data Center Ethernet by Cisco, addresses issues like this and more. To begin, CEE will leverage the speed of 10 GigE to enable the convergence of various data center networks by using a single interconnect technology. Instead of building a separate Ethernet network for the corporate LAN and a FC network for the SAN, CEE will run those systems and others over a single physical cable, resulting in significant hard dollar savings.

CEE also provides priority flow control, which allows different traffic to share a common Ethernet link without interfering with each other. Other features include bandwidth management to ensure consistent QoS, congestion management and data center bridging exchange. The specifications for these capabilities are being worked out by the IEEE and other standards groups.

These features combine to deliver lossless storage, which greatly reduces the likelihood of dropped or lost data packets. For example, priority flow control "lets you pause certain data types, and bandwidth management lets you determine how to allocate the available bandwidth," says Munjal. A storage manager might allocate extra bandwidth to the storage of certain data, such as transactions.

Nelson, at St. Joseph Healthcare, opted for the Sanrad V-Switch 3400 when he needed to combine FC and iSCSI storage. He wasn't looking for converged networking. He just needed a second SAN for disaster recovery and intended to replicate data between the two SANs. Cost, however, played a key factor. "I didn't want to invest in another EMC FC SAN," he notes.

"I wanted synchronous replication, and Sanrad was one of the few vendors that did it," says Nelson. In the process, Sanrad provided lower cost iSCSI storage combined with the hospital's legacy FC storage without actually converging the underlying networks. "Sanrad connects to the arrays via FC but presents to the servers as iSCSI," he continues. That eliminates the need to put costly FC HBAs in each server, using instead a simple Ethernet NIC card.

When it comes to iSCSI, storage managers have choices. They can go with a cheap switch or add features that boost storage performance and reliability but at a higher price. As the industry moves toward converged networks via CEE, the choices and possibilities will only expand.

Alan Radding is a frequent contributor to SearchSMBStorage.com.

This was first published in July 2008

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