iSCSI vs. Fibre Channel - You're Both Right

Reading another article from an expert who provides less than useful information has finally prompted me to try to provide useful guidance for IT managers of 50 to 1,000 diverse servers running a variety of applications.

iSCSI vs. fibre channel (FC) is a classic technology debate with two camps bombarding each other mercilessly with claims that one or the other is right. The reason the debate is so heated and long lived is because there isn't a right answer: there are different situations in which each one is better than the other. Here's how to figure out what's best for you:

Start with the assumption that you'll use iSCSI. It's less expensive, so if it does what you need, it should be your choice. It's less expensive at all levels: The switches and cables enjoy the ecnomy of scale of the massive market for IP networking. You already have staff who know how to manage IP networks. You already have a stock of Cat 6 cables hanging in your server rooms or network closets.

If you have mostly commodity servers, they transfer data to and from direct-attached storage at less than gigabit speeds. Gigabit iSCSI is fine. If you have a lot of servers, you have to size the switches correctly, but you have to do that with FC as well, and the FC switch will be more expensive. Implement jumbo frames so backups go quickly.

Just because you're using iSCSI doesn't mean you're running your storage network over the same cables and switches as your data centre LAN. In fact, you probably aren't. The cost saving doesn't come from sharing the existing LAN, it comes from the lower cost per port and the reduced people cost (skill sets, training, availability of administrators in the labour market) of using the same technology. As long as your storage and general-purpose networks are not sharing the same physical network, a lot of the criticisms of iSCSI evaporate.

If you have large, specialized servers that can and do need to sustain high data transfer rates, then definitely look at FC. Be sure you're measuring (not just guessing) that you need the data transfer rates.

If you have a large farm of physical servers running a huge number of virtual machines (VMs), look at FC. My experience is that virtual machine infrastructures tend to be limited by RAM on the physical servers, but your environment may be different. You may especially want to think about how you back up your VMs. You may not need the FC performance during the day, but when backups start, watch out. It's often the only time of day when your IT infrastructure actually breaks a sweat.

You might look at a FC network between your backup media servers and backup devices, especially if you already have an FC network for one of the reasons above.

Yes, FC will give you higher data transfer rates, but only if your servers and storage devices can handle it, and few today go much beyond one gigabit. FC will guarantee low latency so your servers won't do their equivalent of "Device not ready, Abort, Retry, Ignore?"

The challenge for an IT manager, even (or especially) those like me who have a strong technical background, is that it's easy to get talked into spending too much money because you might need the performance or low latency. The problem with that thinking is that you spend too much money on your storage network, and you don't have the money left over to, for example, mirror your storage, which may be far more valuable to your business.

A final warning: neither technology is as easy to deal with as the vendor would have you believe (no really?). Both will give you headaches for some reason along the way. If it wasn't hard, we wouldn't get the big bucks, would we?