In-Depth
Against the Grain: UltraSCSI Lives Despite New Fibre Channel Kid on the Block
Despite all the fibre channel hype, 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) comes standard with one or two 80MB/s Ultra2SCSI ports, usually right on the motherboard. By adding a single PCI card with dual 80MB/s Ultra2SCSI ports made by one of many vendors, you get an additional 160MB/s peak throughput, or about 142 MB/s sustained – a 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/s 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 160 MB/s per port. The new standard is called Ultra160/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/s Ultra2SCSI devices and the new UltraSCSI 160MB/s devices will operate at full rated speed on the same bus. The industry anticipates that by late 1999 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.
Fibre Channel Misconceptions
Fibre channel will solve all your I/O bandwidth problems. Some vendors of fibre channel claim that 100MB/s port bandwidth is so fast, users will never wait for I/O again. That may have been the case in 1996, when the fastest I/O bandwidth per host adapter was only 40 MB/s UltraSCSI. But with today’s SCSI standard offering at least 60 percent 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 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.
Fibre channel is more reliable than other storage. This has been the greatest misconception. Fibre channel drives run up to 50 percent hotter than identical SCSI drives. In addition, channel fibre 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.
Fibre channel is finally mature and interoperable. 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 from the same vendor. And in most cases, changing one parameter like the host OS, the application or the cable length may cause problems. Several large vendors, such as Quantum and Adaptec, have recently exited the fibre channel market due to interoperability problems. 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 multi-drive servers for the past five years. The technology of SCSI is stable, well-understood, and many vendors have products to fit every price/performance requirement.
Fibre channel is the open standard for the future. This may be the case, but users’ investment in current fibre channel may not be protected. The first fibre storage arrays were introduced by Sun in 1994, based on quarter-Gigabit technology and are only supported on Sun S-Bus servers. This technology is obsolete, and today’s 1-Gigabit fibre channel arrays are not interoperable with this old standard. Vendors are now considering a 2-Gigabit or evenfaster new standard for fibre channel, but there will be no ability to use the current fibre channel drives, hubs, switches or possibly even cables with this new standard.
With today’s Ultra2SCSI, disk arrays can be plug-and-play, backward compatible all the way to SCSI-1 of 1984, and forward compatible to Ultra160/m, which ensures investment protection well into the next five years.
Fibre channel easily supports multi-hosts to form a SAN. Much easier said than done. Without very expensive fibre switches, adding multiple hosts to one fibre channel loop is tricky because the host-software doesn’t support multiple initiators. Often, each host will attempt to reset the channel to gain complete control, which can result in even more lost packets and a hung bus.
In comparison, several vendors make SCSI hardware RAID arrays with multiple, independent SCSI host ports. This allows attaching several servers to one array, each on their own private SCSI bus. This completely eliminates the multi-initiator problem, making implementation far more robust. And the best news: It works today.
About the Author: Jerry Namery is Chief Technology Officer at Winchester Systems Inc. For more information about SCSI, visit the SCSI Trade Association’s site at www.scsita.org.