iSCSI vs. Fibre Channel

Many IT professionals like the concept of storage area networks (SAN), but they're resistant to Fibre Channel (FC), the networking technology that brings nearly all SANs to life. As a protocol optimized for the types of block-mode data transfers used for storage, FC excels at carrying SCSI traffic.

Many IT professionals like the concept of storage area networks (SAN), but they're resistant to Fibre Channel (FC), the networking technology that brings nearly all SANs to life. As a protocol optimized for the types of block-mode data transfers used for storage, FC excels at carrying SCSI traffic. Although the details of the FC protocol are complex and foreign to the IP/Ethernet crowd, they're not difficult for a motivated systems engineer to learn and implement.

But for many IT professionals, the idea of using a local network other than Ethernet, or a protocol other than TCP/IP, seems foreign and archaic. That's why there's so much misguided enthusiasm for IP/Ethernet-based SANs. In nearly every conversation I've had with an IT manager about SANs, the topic swiftly turns to the iSCSI protocol, an emerging IETF (Internet Engineer Task Force) initiative aimed at carrying SCSI traffic over TCP/IP and Ethernet instead of FC. Managers are anxious to begin deploying IP-based SANs as soon as possible.

Yet when pressed to explain their reasoning, IT managers reply, "iSCSI is a lot better than Fibre Channel." "What's wrong with Fibre Channel?" I ask. Amid their hemming and hawing, three main anti-Fibre Channel answers emerge. If you're engaged in the FC vs. iSCSI debate, or are putting off investing in a SAN because you're waiting for TCP/IP-based storage area networking, see if the following apply to you. If they do, wait for iSCSI. If not, then Fibre Channel is still your best bet—and will be for some time to come.

First: Where FC requires that you run a separate fiber-optic network for the SAN, iSCSI lets you leverage your existing Gigabit Ethernet LAN. FC requires a completely separate set of fiber and switches. Still, that doesn't mean the iSCSI network rides for free. iSCSI requires special host-bus adapters dedicated to the SAN, in addition to the server's ordinary Gigabit Ethernet network interface card. That means you'll be using a PCI slot either way. The iSCSI host-bus adapters will consume a Gigabit Ethernet switch port and add to the traffic on your local-area network. Saving the extra fiber isn't much of a benefit as long as you're talking about a short-haul SAN, such as one limited to a server room or large data center.

Second: A server's Fibre Channel host-bus adapters must be connected directly to the SAN switch. An iSCSI host-bus adapter can connect to a storage router (the iSCSI equivalent of the FC SAN switch) anywhere on the Gigabit Ethernet SAN. You will have more flexibility when it comes to building a complex iSCSI-based SAN.

Third: iSCSI is going to be on the Ethernet fast track. While FC is still in the painful process of migrating from 1Gbps to 2Gbps, with 4Gbps the next step, Ethernet is rapidly progressing to 10Gbps and 40Gbps. Plus, Ethernet has a strong tradition of genuine multi-vendor interoperability, and FC does not. These are strong arguments. I've long been a critic of the Fibre Channel's interoperability efforts, and would agree that unless the Fibre Channel Industry Association gets its act together, iSCSI's performance, piggybacked on Ethernet, is going to blow FC out of the water.

But that's the future. For today, iSCSI is still little more than slideware, with only a few vendors showing first-generation wares, like Cisco's SN4250 Storage Router. On the other hand, FC is here, it works, and it can solve real problems today. Although I'm watching iSCSI, my SAN heart still belongs to Fibre Channel.