Counterpoint - Gigabit Ethernet vs. FC for the SAN
nwfusion.com
Water Cooler: Down with Fibre!
By Paul Desmond
Network World, 08/17/98
We've been fairly inundated lately with vendors pushing the idea of storage-area networks. I tried to ignore them at first on the theory that storage is not really a network issue, but these vendors are a relentless bunch. Finally I caved and agreed to meet with a few of them.
I listened to what they had to say and kept stumbling on one thing: Fibre Channel. They all use Fibre Channel technology to link the servers and storage devices that make up a SAN. With Gigabit Ethernet taking off like wildfire, this didn't sit well with me. A lot more people are familiar with Ethernet than with Fibre Channel, which means that if you're going to build a SAN it just seems to make more sense to do it with the former than the latter (If you happen to be a Fibre Channel proponent, please click your way elsewhere now. There's nothing here for you.)
The idea behind storage-area networks is a good one. Instead of having a storage device attached to each network server, you can basically have a farm of storage devices attached to a switch. The switch, in turn is attached to multiple servers, any of which can get at any of the storage devices. Some vendors offer intelligent software that can go fetch data from whatever device it may reside on, then return the whole package to the user. Performance is improved and costs go down, because you can centralize everything. Reliability is also greatly improved, with storage devices backing each other up.
All this has been around in the mainframe world for years. IBM mainframes with ESCON channels attach to a device called an ESCON Director, which in turn attaches to storage devices via more ESCON
channels. Same idea as a SAN.
The latest SAN vendor I met with, StorageTek, tried to answer my questions about Gigabit Ethernet vs. Fibre Channel for SAN applications. They later sent me an e-mail with a laundry list of issues, pointing out why Fibre Channel had an advantage over Gigabit Ethernet for each one. They said Fibre Channel uses a larger packet size than Gigabit Ethernet, making it more suitable for the large file transfers required in SANs. Gigabit Ethernet uses a 1.5K byte packet size, whereas Fibre Channel
assures that up to 64,000 2K bit frames can be sequenced together, enabling for a 128M byte file transfer in a single I/O shot.
They also said Fibre Channel could be stretched over greater distances than Gigabit Ethernet - 10,000 meters vs. 440 meters. And they pointed to the specialized protocols used with Fibre Channel that ensure no data gets dropped.
None of these advantages seemed overwhelming to me. I suspected Gigabit Ethernet vendors could find a way around most of them, if they hadn't already. To find out for sure, I sent the list of comparisons to three Gigabit Ethernet vendors - Alteon Networks, Packet Engines and Extreme Networks.
They refuted virtually every point where Fibre Channel claimed an advantage.
See what they had to say : nwfusion.com
The performance question seemed especially interesting. As an engineer from Extreme Networks put it, "Gigabit Ethernet can deliver one gigabit (1,000M bit/sec) performance with only 1.7% overhead using standard frames, yielding 983M bit/sec of net throughput," he wrote. "Last time I looked, FC topped out at 800M bit/sec. Which one sounds faster?"
The Gigabit Ethernet crowd also voiced the same concern that I had about introducing what, for most users, is a foreign technology.
"The difficulty of implementing Fibre Channel into the core of your backbone networking environment makes an ATM transition look like the easiest thing you've ever done," wrote an Alteon engineer. "Fibre
Channel and HIPPI are, inherently, channel technologies. They really weren't ever developed to be networks."
The distance limitation, meanwhile, seems to be a source of some, shall we say, misunderstanding. Packet Engines says the 802.3z standard calls for Gigabit Ethernet to support distances of up to 5,000 meters between nodes on single-mode fiber. The company has extenders available that can extend the length to more than 80 kilometers.
I'm not entirely sure what to make of this exercise, but at least now I feel comfortable with the whole idea of SANs; they seem to sense and can probably do you some good. I'm still not sold on the whole Fibre
Channel aspect, though. It strikes me that Gigabit Ethernet may already be able to handle SANs adequately and will only get better once Gigabit vendors start addressing the issue head-on. None of them are promising that yet, but consider this my gentle nudge in the effort to get the ball rolling. |
