SAN technology selection: Fibre Channel and iSCSI trade-offs

There are many misconceptions about Fibre Channel SANs and iSCSI SANs, and we'll clear up some of the facts to help you choose the SAN technology that's best for your company.

There are many misconceptions about Fibre Channel SAN and iSCSI SAN technologies. Some of these are inherited from specific storage vendors. Some SAN technology misconceptions are true, but only apply to the past and not today. Other misconceptions are true on a technical basis, but are less relevant in real-world scenarios. This article will clear up some of the most common misconceptions about Fibre Channel and iSCSI SANs, while also helping you choose the best SAN technology for your small and medium business environment.

Myth 1: Fibre Channel SANs are faster than iSCSI SANs

While it is true that 4Gbps Fibre Channel has higher throughput than 1Gbps iSCSI, aggregating four 1Gbps ports together provides the same bandwidth. 8Gbps Fibre Channel has slightly less bandwidth than 10Gbps iSCSI. So from a throughput perspective, this misconception is true for the lower end of the bandwidth spec.

Myth 2: Delayed IOPS is important in SAN technology

Intuitively, iSCSI latency should be much greater than Fibre Channel due to the latency introduced by the TCP protocol. And as latency increases, so should response time. Higher latency generally means fewer IOPS. But VMware's tests of NFS, iSCSI, and Fibre Channel in the fall of 2009 showed some shocking results. In a white paper titled "VMware vSphere 4: Exchange Server on NFS, iSCSI, and Fibre Channel," the test results showed that iSCSI latency was definitely greater than Fibre Channel, especially at low loads. But it also showed that at initial load, Fibre Channel's IOPS was much higher than iSCSI's IOPS and there was a huge difference. Oddly, as the load leveled off over time, the latency IOPS difference narrowed. There was still a small difference, but it was much smaller than you might expect. So unless it was a very high-volume application, latency didn't matter much to small business applications.

Myth 3: iSCSI SANs are always cheaper than Fibre Channel SANs

iSCSI SANs often appear to be cheaper than Fibre Channel SANs. When you compare 1 Gbps iSCSI to 4 Gbps Fibre Channel, iSCSI is significantly cheaper in terms of ownership and maintenance costs. iSCSI hardware, especially at the port level where TCP/IP offload is not required, is also less expensive than Fibre Channel. However, a misleading fact is that iSCSI can be run in an existing infrastructure. While iSCSI can be run in an existing switch and IP infrastructure, it is not recommended. If it is not run on a separate network or subnet (LAN or VLAN), performance is likely to be severely degraded, unstable, and less secure.

But when comparing costs on a per-gigabyte-per-second basis, the difference between iSCSI and Fibre Channel costs is quite low. A 10 Gbps iSCSI SAN is generally more expensive than an 8 Gbps Fibre Channel. This is generally true for initiator ports, target ports, and switch ports. Performance level also makes a difference in cost. At low performance levels (1 Gbps iSCSI compared to 4 Gbps Fibre Channel), iSCSI is a better cost performer than Fibre Channel. At higher performance levels (10 Gbps iSCSI compared to 8 Gbps Fibre Channel), iSCSI is actually more expensive. The difference in operational costs is even more pronounced.

Also, the training costs for iSCSI and Fiber are different. Fiber SAN technology is newer to most storage administrators, so it requires more training costs and a relatively longer learning curve. This is not intuitive. Add to that the manual nature of Fiber Channel, and the operating costs are definitely higher than iSCSI.

In general, the cost of a Fibre Channel SAN and an iSCSI SAN is different for each company. An iSCSI SAN can be more expensive than a Fibre Channel SAN and vice versa, but keep in mind that the cost difference is smaller than you might think.

Myth 4: iSCSI SANs are easier to operate than Fibre Channel SANs

There is no argument that iSCSI SANs are simpler to operate than Fibre Channel SANs because of the non-deterministic, discoverable, and routing nature of TCP/IP Ethernet. Likewise, network implementation, operation, administration, and change management are far more automated and forgiving on iSCSI SANs than on FC SANs. But this conventional wisdom is based on information from the past, not the present.

Recent advances in Fibre Channel SANs for small and medium environments have made them as simple to implement, operate, and manage as iSCSI. Larger environments are still more complex than iSCSI, but change management for Fibre Channel SANs can be handled on a simpler, automated basis with a variety of products from companies including Aptare, NetApp, SANpulse Technologies, and TekTools (now part of SolarWinds). And SAN booting is actually simpler on a Fibre Channel SAN than on an iSCSI SAN (an iSCSI SAN requires at least a separate DHCP server, and typically two for high availability, to provide PXE functionality or boot mirroring).

Which SAN is right for your small or medium business?

There is no one right SAN technology for your small or medium business environment. Both Fibre Channel SANs and iSCSI SANs can do the job. In general, iSCSI SANs appear to be cheaper and simpler; whereas FC SANs are faster, with less latency and higher IOPS. But after reviewing these common misconceptions, you should know that this argument is not always true. The right choice depends on your business's unique needs, skills, experience, and comfort level.