UltraSCSI For Storage Lives On Despite The new Fibre Channel Kid On The Block - Technology Information

Computer Technology Review, Jan, 2000 by Jerry Namery

Despite all the hype about Fibre Channel, the days of SCSI- (Small Computer Systems Interface) based storage are far from numbered. Every new Intel-based server and workstation made today (regardless of price) come standard with one or two 80MB/sec Ultra2SCSI ports, usually right on the motherboard. By adding a single PCI card with dual 80MB/sec Ultra2SCSI ports made by one of many vendors, you get an additional 160MB/sec peak throughput or about 142MB/sec sustained speed, which is considerably faster than what Fibre Channel offers, and that single PCI card supports 30 disk drives. All high performance disk drives from every vendor come in 80MB/sec Ultra2 SCSI configurations at nearly the same price as the slower SCSI models.

That's not all. The industry is rushing to complete yet another doubling in SCSI speed: this time to 160MB/sec per port. The new standard is called Ultra 160/m. Without changing cables or connectors, the doubling in data transfers will be done on the same clock speed (40MHz), but by double-clocking the data. This means that both the current 80MB/sec Ultra2SCSI devices and the

new UltraSCSI 160MB/sec devices will operate at full rated speed on the same bus. The industry anticipates that, soon, nearly all SCSI disks and Intel-based servers will come standard with Ultra160/m port. The storage industry sees a rosy future and it's Ultra160/m.

DISPELLING THE MISCONCEPTIONS ABOUT FIBRE CHANNEL

Common Fibre Channel misconceptions abound, but if you read on, you can take the mysticism out of the myths.

* Fibre Channel will solve all your I/O bandwidth problems.

Some vendors of Fibre Channel claim that 100MB/sec port bandwidth is so fast that users will never wait for I/O again. That may have been in 1996, when the fastest I/O bandwidth per host adapter was only 40MB/sec UltraSCSI, but with today's SCSI standard offering at least 60% more bandwidth per PCI host adapter at a lower cost, most users select higher bandwidth.

* Fibre Channel lets you put up to 126 hosts and storage devices on one loop.

In practice, users find that, to maintain reliability, they should limit the number of drives per loop to a few dozen. Otherwise, they may experience hung loops or even lost data. The problem may stem from loop arbitration (Fibre Channel arbitrated loop) or because every drive must pass the loop to the next. So, the advantage of fibre versus SCSI may be lost: one new PCI adapter with dual Ultra2SCSI supports 30, about the same as Fibre Channel.

* Fibre Channel is faster for all applications.

Fibre Channel storage is not well suited for most business applications, since FC-AL arrays usually require the host to perform RAID 5. Using any host-based RAID 5 is unacceptable to most users of high performance servers, since reliability and performance are dramatically reduced over hardware RAID 5. In general, Fibre Channel storage has been well received by users of video and imaging applications where host-based RAID-0 or striping is acceptable. In these applications, users like the ability of Fibre Channel to support longer distances and run over fiber optic cables.

* Fibre Channel is more reliable than other storage.

This has been the greatest misconception. Fibre channel drives run up to 50% hotter than identical SCSI drives. In addition, Fibre Channel is a network protocol that relies on Class 3 data transfers, which means coping with lost data packets. Since Class 3 service relies on the application to recover from a lost packet, the result to users' data may be catastrophic. Today's applications were written to support reliable SCSI data transfers. SCSI is a parallel bus technology, similar to the buses that make up all computer systems, and every transfer is guaranteed and acknowledged with proper handshakes. Fibre Channel a serial data channel designed to deliver 1Gbit of information over long distances without the necessary hardware error recovery.

* Fibre Channel is finally mature and interoperable.

The problem of Fibre Channel interoperability, the ability of different vendors' products to work with each other, especially under varying conditions, is still being resolved. Interoperability is even a problem between different equipment of the same vendor and, in most cases, changing one parameter like the host OS or the application or the cable length may cause problems. Several large vendors have recently exited the Fibre Channel market due to interoperability problems such as Quantum and Adaptec. Both these vendors announced their intention to focus all their energies on Ultra2SCSI and the upcoming Ultra160/m standard.

* Fibre Channel storage is very similar to SCSI-based RAID arrays.

Most Fibre Channel arrays sold are actually JBOD ("Just a Bunch Of Disks") and most users of Fibre Channel use them in JBOD or host-based RAID 0. This is because vendors found it extremely challenging and expensive to make Fibre Channel hardware RAID work. SCSI-based hardware RAID arrays have been the staple of the storage industry for nearly every server requiring more than a few drives for the past five years. The technology of SCSI is stable, well understood, and many vendors have fine products to fit every price/performance requirement.