Fibre Channel and iSCSI: Dueling storage networking protocols

You've probably heard the mantra that Fibre Channel SANs are used at big companies while iSCSI SANs are good for SMBs. But why? Drill down into the cost and performance differences between Fibre Channel and iSCSI so you can guide your customers toward the right choice.

By Yuval Shavit, Features Writer

If you are designing a storage area network (SAN) for your client, one of the first decisions you'll have to make is how to connect the SAN's component drives. The two main ways of doing this are Fibre Channel and iSCSI, with a third option, Fibre Channel over Ethernet (FCoE), emerging as a bridge between them.

Essentially, Fibre Channel (FC) is the faster but more expensive option, while iSCSI is cheaper to build and maintain but is slower and can impact the rest of your local area network (LAN). FCoE uses the Fibre Channel protocol and iSCSI's hardware, thus addressing some of the performance concerns of iSCSI while avoiding the infrastructure costs of FC.

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Which technology you should use depends on your client's needs, said Greg Schulz, founder and senior analyst with The StorageIO Group, a consulting firm in Stillwater, Minn. Generally, companies that need highly scalable, high-performance systems should use FC, but iSCSI will be the "sweet spot" for many small and medium-sized businesses (SMBs), Schulz said.

Cost of Fibre Channel and iSCSI

The biggest inhibitor to FC, and the reason that SANs have traditionally been used only in larger enterprises, is its cost. A Fibre Channel network uses different connectors and wires -- which may be twisted copper or optical -- from Ethernet, so a company that's installing a new SAN will have to invest in a dedicated FC network for the storage devices.

FC equipment is more expensive than Ethernet. According to a study by Forrester Research, each adapter that connects a storage device to the FC network will cost $1,100 to $1,400. FC cables themselves cost $50 to $150 for 20 feet, instead of about $12 for Category 6 Ethernet, and your client will also have to buy separate switches for an FC SAN. Those cost about $650 to $1,500 per port, the study found, whereas a Cisco Catalyst 6500 Ethernet switch costs about $250 per port.

Maintaining FC infrastructure will require at least an additional investment in monitoring tools and training, since FC doesn't use the Internet Protocol (IP) that most network administrators are already familiar with, said Henry Baltazar, storage analyst for The 451 Group in San Francisco.

iSCSI reduces those costs by connecting devices using Ethernet hardware. It works by wrapping SCSI commands in IP packets, thus enabling it to transmit block-level data over a client's existing LAN.

Fibre Channel and iSCSI performance

Because iSCSI runs over Ethernet, its performance is potentially limited by your client's LAN speed and other traffic. iSCSI also has higher latency, since each SCSI packet needs to be wrapped in an IP packet. But with 10 Gigabit Ethernet (GigE), the gap between Fibre Channel and iSCSI is shrinking, said Paul Franco, executive vice president at Zibiz Data Management, a storage consultancy in Ronkonkoma, N.Y.

Fibre Channel generally operates at 2 to 4 Gbps, Baltazar said, although faster hardware can reach up to 8 Gbps. But because FC runs on a dedicated network, it is able to use all of that bandwidth for the SAN; since iSCSI runs on the LAN, other data on the network can slow the SAN down. Conversely, too much iSCSI traffic can flood the network and slow down other operations, so it's a good idea to create a virtual LAN (VLAN) for the SAN, Baltazar said. Because you can allocate bandwidth for different VLANs on your network, isolating SAN traffic to one can ensure that it has all the bandwidth it needs without interfering with other parts of the network.

While the differences between Fibre Channel and iSCSI are clear, the emerging FCoE spec promises to muddy the waters a bit. Although it is still in development, FCoE is designed to fit between Fibre Channel and iSCSI by transmitting the FC protocol directly over Ethernet, without IP. This will allow companies to use 10 GigE while keeping existing FC infrastructure. FCoE's hybrid nature allows companies to transition from FC to iSCSI, Schulz said. Most major networking and storage vendors back FCoE, which is being developed by the same group responsible for Fibre Channel standards, the International Committee for Information Technology Standards' Technical Committee T11.

In the next installment of our Storage Area Network Design Services Hot Spot Tutorial, we'll drill down into key SAN design considerations.

This was first published in December 2008

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