SCSI vs. Fibre Channel Drives
By Pat McGarrah
The debate is raging over the future method of attaching hard drives to storage subsystems. Fibre Channel-Arbitrated Loop (FC-AL) is threatening to unseat parallel SCSI. Meanwhile, a new version of SCSI--Ultra160/m (a subset of the Ultra3 SCSI specification)--is emerging. (For more information on Ultra160/m, see News)
FC-AL proponents argue that Fibre Channel is becoming a de facto standard for storage networks. So, doesn`t it make sense to apply this new serial technology to disk drives? No, say Ultra160/m advocates, because the computer industry was built on backward compatibility.
There is no problem with Fibre Channel at the network level. Implemented as either FC-AL or as a switched fabric, Fibre Channel provides the industrial-strength connectivity and cable-length support needed for "box-to-box" interconnects. A prime example is connecting large numbers of servers, clients, JBOD, and RAID subsystems over long distances in storage area networks (SANs), the storage equivalent of LANs.
At the hard-drive level, however, it`s a more difficult decision. For some high-end storage applications, drives that support FC-AL (currently the only Fibre Channel technology implemented as a drive-level interface) miss the mark. They simply cannot meet two critical requirements for enterprise-class storage: easy data management and backward compatibility. So, the dominant high-speed interface employed "inside the box" will likely remain parallel SCSI for quite some time. The hard drives in most boxes will be a mix of Ultra2 SCSI and Ultra160/m devices. The Ultra160/m interface features new data management capabilities and a bus transfer rate of 160MBps.
The notion that FC-AL isn`t a universal solution for all data-intensive storage and interconnect requirements may still take you by surprise. If so, there`s good reason. Early reports about Fibre Channel products often erroneously used "FC-AL" as a catch-all phrase for Fibre Channel implementations, blurring the distinctions between switched fabric and FC-AL on the one hand and interconnect-level and drive-level interfaces on the other. These reports also focused on the connectivity and cable-length advantages of FC-AL as if they were the prime considerations for all storage applications, along with high data throughput. Nor did they anticipate the arrival of LVD (low voltage differential), starting with Ultra2 SCSI. This signaling technology overcomes the cable-length restrictions associated with earlier SCSI drives (allowing cable lengths of up to 12 meters), while providing fast transfer rates and minimal migration costs. Finally, the FC-AL fanfare didn`t take into account Ultra160/m development.
The upshot: Sales of first-generation FC-AL drives have been slow to take off, and market analysts don`t expect them to capture much of parallel SCSI`s traditional turf anytime soon.
So, what`s the problem with FC-AL drives? For one, they sit on a single loop rather than on a storage bus, and the loop inherently cannot support mixed drive speeds.
That means if hard drive vendors unveil 2Gbps FC-AL drives next year and a company wants to add those drives to a RAID or JBOD box equipped with current 1Gbps devices, all the drives on the loop will drop down to the lower speed. And that raises data management problems for IT organizations.
Consider two options for solving the mixed-speed problem. One option is to keep things simple by throwing out all previous-generation FC-AL drives each time a next-generation drive comes out. You will benefit from the new drive`s faster speeds; however, there is a considerable cost penalty--one that only highly specialized, niche storage markets or the most well-heeled companies could justify. Such issues will be worked out in time, but for now, such potential problems have slowed acceptance of FC-AL drives.
The only alternative is to attach different-generation FC-AL drives to different loops within a single cabinet. The main loop runs the fastest drives and a switch is added to each of the auxiliary loops so all loops operate as a single storage system. Adding these switches is costly, but that`s not all. Whenever IT staffers want to add or replace a drive within a particular box, they would have to know the drive`s speed to determine which loop it should be attached to. This information is not obvious. Given the proliferation of external storage boxes in organizations today, as well as the frequency of hard-drive installations and replacements, this requirement would swiftly add up to a significant data management challenge.
"Forward friendly" SCSI LVD solves the problem because it can support mixed drive speeds on the same parallel bus. Incorporating future Ultra160/m hard drives into existing Ultra2 SCSI LVD storage infrastructures will not pose a data management problem because the SCSI initiator automatically communicates with all the devices on the bus and negotiates the data transfer rate to support. As a result, data transfers between an Ultra160/m drive and an Ultra160/m adapter will be able to zip along at 160MBps, even with Ultra2 SCSI drives plugged into the bus. Transfers using the Ultra2 SCSI drives will proceed at the Ultra2 SCSI rate.
The new Ultra160/m interface takes data management a step further with a technology called domain validation. This technology not only doubles the Ultra2 SCSI transfer rate, it also intelligently tests the entire storage network, including legacy systems` cable plants, to ensure the network can support the Ultra 160/m transfer rate. If it can`t, data transfers proceed at a lower rate, much the way today`s modem and fax transmissions connect despite variations in equipment. The technology keeps drive systems up and help-desk costs down as companies scale to higher-performance, higher-capacity SCSI solutions.
Parallel SCSI also has an edge in terms of backward compatibility, infrastructure support, easier implementation for OEMs, and lower overall cost. That`s why many industry observers believe SCSI drives "inside the box" and Fibre Channel interconnects "outside the box" will be the trend for SANs, digital video, RAID, and other high-end storage applications. The infrastructure for such hybrid solutions is developing rapidly. Today`s RAID controllers typically support both Fibre Channel interconnects box-to-box and SCSI drives in the box, and SCSI-to-FC-AL bridges for JBOD storage are also available.
Other scenarios could emerge, however. Fibre Channel as a serial interface for hard drives is extremely interesting. Serial technology should simplify the connection of drives to a storage system, as well as support very long connecting cables.
So, the question actually revolves around when Fibre Channel will have a shot at becoming the dominant choice for drives inside the box. As long as parallel SCSI continues to evolve, the issue will be one to watch.
Pat McGarrah is program director of technology at Quantum Corp., in Milpitas, CA.