A preview of Fibre Channel performance
Today's environment demands
very fast transfer of large volumes of information. No wonder the Fibre Channel (FC) and Fibre Channel Arbitrated Loop (FC-AL) storage interfaces have generated
so much interest. If you compare SCSI's current high-end 40MBps data transfer
rate with FC's high-end 106MBps, you might think you can realize a 265 percent
improvement in performance. But is FC that much faster? The Windows NT
Magazine Lab compared the two interfaces.
ANSI developed the FC Standard in 1988 as a practical, inexpensive, and
expandable method of using fiber optic cabling to transfer data among desktop
computers, workstations, mainframes, supercomputers, storage devices, and
display 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 a
nine-pin type connector, as you see in the photo. So despite the name, most
current implementations of FC don't use fiber optic cabling.
Copper wire connections work for up to 30 meters; beyond that distance, you
must use fiber optic cabling and connectors. With fiber optic cables, you plug
in an optic converter on both ends of the FC connection, and you can reach up to
10km. This distance is clearly superior to the current limitation of 25 meters
for the differential SCSI technology. (For more information about SCSI, see Sean
Daily, "SCSI and IDE: Defining the Differences," June 1997.)
FC's design is similar to the Open Systems Interconnect (OSI) network
layers. FC supports several data communication protocols, including Fiber
Distributed 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, and
106.4MBps; and it can achieve these transfer rates in both directions
simultaneously (duplex). Thus, FC can transfer data at more than
200MBps, if usage is balanced in both directions--quite an improvement over the
simplex and half-duplex interfaces such as SCSI. Work is under way for FC
specifications of 400MBps (again, in both directions simultaneously; this
technology could attain a data rate of more than 800MBps). Also note that an
Ultra2 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 for
Windows NT) is FC-AL, which ANSI developed to connect peripherals. In FC-AL, you
usually 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. An
FC-AL natively uses the SCSI-3 (SCSI FCP) protocols and can address 127 FC
devices or nodes within the limitations of the 30-meter copper cable or the 10km
optic cable. (A SCSI connection can handle only seven devices, excluding the
computer.)
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. You
run a cable from the card in the computer to the first FC device and then
connect each FC device in the chain to the next device. On the last device in
the chain, you plug a loop-back connector into the out connector, which runs the
data 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 a
balanced negative and positive wire pair to improve data integrity and to let
you spread the network over greater distances.
FC and NT
Several vendors support FC in the NT market. Adaptec and Emulex have FC
cards 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, and
Seagate FC-AL hard drives. Some manufacturers are producing hard drives for use
in FC arrays. Unfortunately, no hardware-based FC RAID solutions are currently
available. If you want RAID, you must use the slower, software-based RAID that
is included with NT Server.
The Lab compared Raidtec's FC offering with its SCSI-3 RAID system, using
an Amdahl 200MHz quad-processor Pentium Pro system with 512MB of RAM. Because
hardware-based RAID isn't available for FC, we tested both systems using a
four-drive stripe set, which writes the data evenly across all the drives
without fault tolerance. To get a general idea of the difference in transfer
speed, we timed copying NT's Service Pack 3 from one directory to another
directory on the same disk. The FC system was about 5 seconds faster.