How fast Is Fibre Channel?
By Zachary Shess
While still an emerging technology in terms of actual implementation, Fibre Channel awareness within the storage industry is running high. While Fibre Channel`s ability to support multiple protocols and longer cabling distances are important assets, everybody wants to talk about its speed. After all, this storage/network interconnect can provide data transfer rates of 100MBps, right?
Well, it depends. "Real world" data transfer speeds within a Fibre Channel-based network are generally not congruent with vendor specs or the 100MBps ANSI specification. While that conclusion may not warrant a call to the Independent Counsel, industry experts suggest that integrators and end-users looking to incorporate Fibre Channel may want to consider several important factors, in particular the type of application and network overhead/latency.
Mike Kane, director of Fibre Channel marketing for Emulex, shares the opinion of others: Benchmarks are only useful after you know what applications were run in the tests.
Frequently, to obtain the optimum speed for Fibre Channel products, benchmark testing is performed using just enough disk drives to saturate the network. This laboratory approach often does not reflect real-life network environments, where scenarios and requirements vary. A streaming video application, for example, may require full Fibre Channel speed to move large data-intensive files in a reasonable time. Conversely, an OLTP application may require rapid I/O to facilitate thousands of requests for data.
Experts suggest conducting individual performance tests for every type of application you plan to run on the network. "If I`m running a video editing workstation, the MBps numbers are probably pretty good because I`m moving data," Kane says. "but if I`m running a database server, then those numbers may be useless."
If 100MBps is Fibre Channel`s gross throughput, network overhead is the tax that`s subtracted to make the net total. Often measured in milliseconds, overhead is the time for the CPU to send out a command through the network, have the disk drive access the information, put the data onto the Fibre Channel interconnect, and send it to the client. Generally, additional time is taken up by other network activities, such as traffic monitoring and data encoding.
Fibre Channel topologies can affect throughput rates, according to Steve Bucher, president of I-Tech, a vendor of Fibre Channel and SCSI protocol analyzers. "If you`re talking networks, you`re likely starting with a switched fabric environment, where the bandwidth of the entire system generally increases as the number of ports increase," Bucher explains. "But with an arbitrated loop environment, you have a single loop with a fixed bandwidth. The more devices you have, the more they have to share the fixed bandwidth." So, as the number of devices increase in an environment, speed is limited by the topology.
"In the storage environment, where you have a fixed bandwidth within the arbitrated loop, and you add a lot of devices, initiators, and targets, you may run into bandwidth limitations. In a switched environment, your bandwidth is determined by how much your switch can support and how many device connections it can support," Bucher adds.
Not surprisingly, it is difficult to provide an absolute number. Under certain conditions, Kane says users can expect up to 85MBps to 90MBps.
But those estimates may represent best-case scenarios. "Box to box, you may achieve 80MBps, but with a network, it can be slower. It just depends on your configuration," says Jan Dedek, founder of Ancot Corp., a network protocol analyzer vendor.
Mike Harvey, a sales support engineer with Xyratex, another Fibre Channel analyzer vendor, has seen speeds as high as 90MBps in testing done with SCSI-based disk drives over a Fibre Channel network. Fibre Channel, he says, is really a transport mechanism, so the key is "how fast data can be pulled off the media and plugged into the wire."
John Adam, vice president of marketing at Finisar Corp., another analyzer vendor, views it similarly, but pinpoints the loss of speed on the software side.
"If you just look at the hardware design, Fibre Channel seems to be capable of sustaining 80MBps to 90MBps. The issue is primarily related to the STL [SCSI Transfer Layer] software one layer up--how well it`s written and how much caching is going on," Adam says. "In reality, it`s how many layers you have to go back to, to get the data and move it on to the network. It`s like a train: You know the train`s speed. It`s a matter of getting information on to the train."
Other factors to consider when testing Fibre Channel speeds include the number of clients accessing data, the ability of the host adapter to get the data on to the Fibre Channel network, and the number and speed of disk drives.