The Future of SCSI
Enhancements such as Ultra2 SCSI, expanders, LUN bridges, switches, and Ultra3 SCSI promise to expand the life of the venerable interface.
By Harry Mason
The SCSI interface has been the workhorse of the server and workstation markets for nearly 15 years. And the cabling and device addressing limitations of the various implementations of IDE have allowed SCSI to remain the predominant interface for out-of-box peripheral connectivity in the desktop market. In recent years, numerous alternatives have been suggested and offered as replacements for this ubiquitous interface. So, it`s important to reassess what markets SCSI will continue to serve and to project where the interface will be positioned within the storage industry over the next several years.
One of the more recent SCSI enhancements is the Ultra2 specification. Borrowing from a point-to-point modem standard (EIA-644) called Low-Voltage Differential Signaling (LVDS), the SCSI community derived an LVD implementation, commonly referred to as Ultra2 SCSI (see table for naming conventions and key specs).
Ultra2 LVD extends SCSI cable lengths to 12 meters and is addressable up to 16 devices. These advances, coupled with the doubling of performance to 80MBps, allow system manufacturers and storage integrators to leverage their existing software and knowledge base into a new generation of SCSI systems with higher performance, improved signal integrity, greater "addressability," and enhanced configuration flexibility.
LVD is the enabling silicon technology that fueled these enhancements; however, improved terminators, cables, connectors, and new connectivity schemes also enhanced the interface. For example, the miniaturization of Very High Density Connector Interfaces (VHDCI) now allows up to four Ultra2 SCSI ports to be connected to one PCI bus load, providing 320MBps of aggregate I/O bandwidth. Although this bandwidth may exceed the useful bandwidth of 32-bit PCI systems, transaction processing environments are more often limited by device addressability than raw bandwidth, which is also addressed by Ultra2 configurations. Such configurations provide high performance, concurrent operation between multiple buses, and extended addressability up to 60 devices--all with a single PCI bus load.
To address the reliability concerns of configuring these types of systems, companies no longer have to cable drives together in their systems. Instead, the Single Connector Assembly (SCA2) can be used in backplane environments to substantially reduce or eliminate the need for cabling between drives.
These larger systems often need to be distanced or isolated from the servers. In configurations such as Windows NT fail-over clustering, failed nodes in a cluster may need to be shared or isolated. A relatively new type of connectivity device, called an expander, allows SCSI buses to be separated at distances substantially farther than 12 meters (see figure on p. 35). Expanders can isolate failed components, allowing for continuous operation during system failures or maintenance.
SCSI distances have traditionally been limited by the electrical properties of the single-ended connection. Capacitive loads, device spacings, stub lengths, cable dialectrics, and silicon signaling rates affect SCSI cable lengths. The theoretical limit to SCSI`s maximum distance is actually determined by the ARBitration Delay specification of 2.4 microseconds, or the amount of time SCSI devices have to gain control of the bus to participate in the arbitration process. Depending on the cabling environment and the dialectrics of the cabling, the theoretical distance limit can be as long as 50 meters. By deploying expanders that electrically isolate the various bus segments, integrators can "stretch" SCSI distances to tens of meters, in much the same way that Ethernet repeaters are used (see figure above).
Because expan-ders inherently isolate separate segments of the SCSI bus, they can be used to mix existing single-ended and high-voltage differential SCSI with LVD bus segments, providing investment protection.
With drive capacities surging at 25% to 30% per year, the cable distance, addressability, and reliability provided by up to four Ultra2 SCSI buses--especially with expanders--should more than meet desktop, workstation, and server storage demands for the foreseeable future. However, certain enterprise environments will require significantly more in the way of addressability, connectivity, and distance. In these environments, devices such as Logical Unit Addressing (LUN) bridges and SCSI switches can be used.
LUN bridges take advantage of SCSI`s LUN feature so that a single SCSI bus can address another SCSI bus in much the same way it would address another SCSI peripheral. In the simplest of configurations, LUN bridges can address 225 devices (15 SCSI devices on 15 SCSI buses). More complex implementations could theoretically address more than a thousand devices from one SCSI bus.
Whereas LUN bridges extend SCSI`s addressability limits, SCSI switches offer connectivity and distance advantages. Switches can be used to aggregate multiple SCSI buses into a large SCSI network (see figure). SCSI switches offer virtually unlimited addressability and can operate in conjunction with serial connections. The ability of switches to expand SCSI networks provides users with a relatively easy way to scale systems, without deploying new technologies.
Universal Serial Bus (USB), Serial Storage Architecture (SSA), 1394, Fibre Channel, Integrated Drive Electronics (IDE), and yet to be delivered interfaces all compete with SCSI in one way or another. But SCSI is taking on some of the attributes of the emerging interfaces.
One thing that fundamentally distinguishes SCSI from competing technologies is its bus-a distributed transmission line. This feature has many benefits, despite bandwidth limitations. For example, SCSI devices can always operate at their fundamental capability, independently of other devices attached to the bus. Fast SCSI, Ultra SCSI, and Ultra2 SCSI devices coexist in a common bus environment and operate with the same software, allowing users to protect their initial investments, to upgrade to the next level of SCSI capability at their own pace, and to maintain cross-generation compatibility.
New standards such as Fibre Channel leverage SCSI`s logical command structure, but they do not leverage the millions of lines of software designed to manage the physical topologies of SCSI`s distributed transmission line. Several hundred man-years of SCSI RAID code exist to provide topology management, and that`s not likely to be displaced soon.
Another benefit of a distributed transmission line can be drawn from a comparison of this interface with serial architectures. FC-AL, for example, requires loop resiliency circuits (or port by-pass circuits) to maintain the integrity of loops when individual drive components fail. This means added costs. Distributed transmission lines, in contrast, do not. In addition, users of high-availability systems often want the backplanes that connect disk drives to be free of active components. In these cases, port by-pass circuits must be configured in customer-replaceable units, adding still further costs, increasing complexity, and potentially creating reliability issues.
Storage analysts agree that SCSI will be the primary disk drive interface for the server industry for several years. A recent report by Dataquest, a market research firm in San Jose, CA, predicts that FC-AL shipments will not exceed SCSI shipments until 2003. Fibre Channel features such as dual porting, while significant in value, add complexity and cost to storage controllers and complicate interoperability issues.
In the high-end server and storage-area network environments, however, SCSI`s market share is expected to erode, thanks primarily to Fibre Channel. In these markets, Fibre Channel`s speed and distance capabilities will be significant and promise to make Fibre Channel a much more prevalent interface in the future.
However, those that predict SCSI`s early demise in this market may be missing a number of important points. Markets that require technologies such as Fibre Channel (e.g., the enterprise) have been characteristically slow to adopt new technologies, especially when higher priority concerns such as year 2000 issues are looming. Furthermore, even users who are deploying Fibre Channel to connect to external storage cabinets are typically using SCSI drives internally. These factors, coupled with the emerging channel assembly model, mean that interfaces must provide a high degree of interoperability, serviceability, ease of configuration, multiple sources, and competitive prices.
The scalability advantages promised by emerging interfaces may become less significant as 8-way and 16-way Pentium Pro, Pentium 2, and Merced-class systems are delivered. Some analysts predict that two-way fail-over clusters will be the primary configuration for clustering, as opposed to performance clustering with high node counts. The many-hosts-to-many-storage-subsystems model may not be as prevalent as expected due to powerful processor complexes and rapid advances in storage capacities.
In the desktop arena, SCSI is being challenged primarily by IDE, USB, and 1394. With the advancements in UltraDMA, IDE promises to hold a strong position in the desktop market as the primary internal interface for years to come. But the market for connecting outside the system remains unsettled. USB, after a slow start, is penetrating the relatively low-performance peripherals market. The 1394 interface has experienced some recent setbacks, with a number of prominent vendors scaling back their commitments. In addition, moving 1394 from a separate clocking mechanism to an embedded clocking mechanism has created a number of technical hurdles.
While several new interfaces will eventually establish critical mass, improve interoperability, and cost competitiveness, SCSI will continue to evolve. The industry is in the process of deploying many of SCSI`s relatively new capabilities and has yet to fully realize the potential of Ultra2, bus expanders, LUN bridges, and switches. And even more improvements to SCSI are under way. For example, development activity for the next generation--Ultra3 SCSI--has begun. Ultra3 promises another doubling of performance to 160MBps, while leveraging the installed base of software.
Although a few emerging interfaces will garner significant roles in the storage interface market, clearly SCSI will continue to evolve and will play an important role. With its ability to leverage the installed software base, SCSI is expected to support massive amounts of storage and peripheral connectivity solutions for the foreseeable future.
Expanders allow integrators to separate SCSI buses at distances of more than 12 meters.
SCSI bus expanders can be used to add distance, connect additional bus devices, provide electrical isolation between two environments (e.g., bused and cabled), or to mix dissimilar SCSI signalling schemes (e.g., single-ended and LVD).
SCSI switches can aggregate multiple SCSI buses into a SCSI network to expand scalability and cable distances between devices.
Harry Mason is director of strategic marketing for Symbios` client & server products division. He is also chairman of the SCSI Trade Association (www.scsita.org).