Fibre Channel, SCSI and You

A preview of Fibre Channel performance

Today's environment demandsvery fast transfer of large volumes of information. No wonder the Fibre Channel (FC) and Fibre Channel Arbitrated Loop (FC-AL) storage interfaces have generatedso much interest. If you compare SCSI's current high-end 40MBps data transferrate with FC's high-end 106MBps, you might think you can realize a 265 percentimprovement in performance. But is FC that much faster? The Windows NTMagazine Lab compared the two interfaces.

ANSI developed the FC Standard in 1988 as a practical, inexpensive, andexpandable method of using fiber optic cabling to transfer data among desktopcomputers, workstations, mainframes, supercomputers, storage devices, anddisplay devices. ANSI changed the standard to support copper cabling; today,some kinds of FC use two-pair copper wire to connect the outer four pins of anine-pin type connector, as you see in the photo. So despite the name, mostcurrent implementations of FC don't use fiber optic cabling.

Copper wire connections work for up to 30 meters; beyond that distance, youmust use fiber optic cabling and connectors. With fiber optic cables, you plugin an optic converter on both ends of the FC connection, and you can reach up to10km. This distance is clearly superior to the current limitation of 25 metersfor the differential SCSI technology. (For more information about SCSI, see SeanDaily, "SCSI and IDE: Defining the Differences," June 1997.)

FC's design is similar to the Open Systems Interconnect (OSI) networklayers. FC supports several data communication protocols, including FiberDistributed Data Interface (FDDI), High-Performance Parallel Interface (HiPPI),Intelligent Peripheral Interface (IPI)-3, Internet Protocol (IP), SCSI-3,Ethernet, Token Ring, and asynchronous transfer mode (ATM).

You can conFigure FC for data rates of 13.3MBps, 26.6MBps, 53.2MBps, and106.4MBps; and it can achieve these transfer rates in both directionssimultaneously (duplex). Thus, FC can transfer data at more than200MBps, if usage is balanced in both directions--quite an improvement over thesimplex and half-duplex interfaces such as SCSI. Work is under way for FCspecifications of 400MBps (again, in both directions simultaneously; thistechnology could attain a data rate of more than 800MBps). Also note that anUltra2 SCSI version is under development that will offer speeds up to 80MBps.

FC Configurations
You can conFigure FC ports three ways: in an FC-AL, in point-to-point links,or in a switch. The most common configuration (and the one you can buy now forWindows NT) is FC-AL, which ANSI developed to connect peripherals. In FC-AL, youusually connect the output of one FC device to the input of another FC device,and you connect the last device back to the first device, creating a loop. AnFC-AL natively uses the SCSI-3 (SCSI FCP) protocols and can address 127 FCdevices or nodes within the limitations of the 30-meter copper cable or the 10kmoptic cable. (A SCSI connection can handle only seven devices, excluding thecomputer.)

FC-AL is a simple closed serial loop. An FC-AL device has two connectors(one in and one out) an arrangement that makes connecting devices a breeze. Yourun a cable from the card in the computer to the first FC device and thenconnect each FC device in the chain to the next device. On the last device inthe chain, you plug a loop-back connector into the out connector, which runs thedata from the connector's send side to its receive side to complete the loop.Each connection uses four wires: two for transmitting and two for receiving.Using an electrical differential technique, FC (like differential SCSI) uses abalanced negative and positive wire pair to improve data integrity and to letyou spread the network over greater distances.

FC and NT
Several vendors support FC in the NT market. Adaptec and Emulex have FCcards with drivers for NT, and Raidtec has a complete FC hard drive setup:Raidtec hard drive enclosure, an Emulex LightPulse PCI FC Host Adapter card, andSeagate FC-AL hard drives. Some manufacturers are producing hard drives for usein FC arrays. Unfortunately, no hardware-based FC RAID solutions are currentlyavailable. If you want RAID, you must use the slower, software-based RAID thatis included with NT Server.

The Lab compared Raidtec's FC offering with its SCSI-3 RAID system, usingan Amdahl 200MHz quad-processor Pentium Pro system with 512MB of RAM. Becausehardware-based RAID isn't available for FC, we tested both systems using afour-drive stripe set, which writes the data evenly across all the driveswithout fault tolerance. To get a general idea of the difference in transferspeed, we timed copying NT's Service Pack 3 from one directory to anotherdirectory on the same disk. The FC system was about 5 seconds faster.

For a more sophisticated test, we used BlueCurve's Dynameasure benchmarkingtool to test the networks' performance. (For more information about this tooland the benchmarks we used, see Carlos Bernal, "Dynameasure Enterprise 1.5by BlueCurve," page 81.) Because we were working with alpha and beta FCproducts, we kept our tests simple and used the SQL and File transfer benchmarksto look only at whether FC offers a performance advantage.

For the SQL benchmark, we used the Order Entry/Single Read/Writeand Order Entry/Mixed Read/Write tests with a 96MB dataset. For the Filebenchmark, we used the Copy All Bi-directional benchmark, which copies arandom mix of files; the file dataset size was 35.6MB. We used 20 client systemsin a five-step test that simulated 20 to 100 users.

FC and SCSI-3 performed about the same in the SQL test, as you see in Table1. In the file transfer test, we found FC to be 10 percent to 20 percent fasterthan SCSI-3, as you see in Table 2. This result is much different from the 265percent that you might expect. Of course, performance is subject to change withthe final releases of the hardware components and software drivers.

So, why wasn't FC faster? Perhaps the hard drives couldn't spit out datafast enough, or the FC card in the server couldn't pump data into memory fastenough, or the CPUs couldn't take the data out of memory and dump it back to theFC card fast enough, or a combination of these factors. This situation isanalogous to the difference between a 10Mbps and a 100Mbps Ethernet: Theperformance doesn't increase 10 times when you move from 10Mbps to a 100Mbpsnetwork, only about 2.5 times. The speed of the wire (the transfer rate) is onlyone of many factors that come into play when you interconnect equipment.

Today, FC can connect more devices and give you an extra bit of performanceover SCSI-3, although you can approach FC's speed and connectivity by usingmultiple SCSI controllers. FC costs more now because it is new technology, butits costs are already coming down. If you need the fault tolerance of RAID, youcan buy hardware-based SCSI RAID controllers, which are much faster thansoftware-based RAID, thus making a multichannel SCSI solution a better solutionthan FC for many applications. But tomorrow will bring hardware-based FC RAIDsolutions and FC-to-SCSI converters. Then FC will be more attractive.