Ultra160/m Will Become The Preferred SCSI Implementation - Technology Information

Computer Technology Review, July, 1999 by Michael Joyce

Michael Joyce is the director of channel marketing at Mylex (Fremont, CA).

The arrival of Ultra3 SCSI and Ultral60/m SCSI builds on a long tradition of improvement in the SCSI specification and upon the lessons learned from installing over 300 million parallel SCSI products since the first SCSI specification was released. This article reviews SCSI-to-date and looks at the changes Ultra3 SCSI and Ultra160/m bring to next generation storage devices. Most importantly, this article explores what's in it for the end user.

The SCSI-3 specification has many sections. The part that codifies the SCSI Parallel Interface specification (SPI-3) is where Ultra3 SCSI is defined.. This document requires compliant product to have any one or any combination of the enhancements defined in the specification. The enhancements are double transition clocking, domain validation, cyclic redundancy checking, packetization, and quick arbitration and selection.

These features increase throughput to rates up to 160MB/sec. and improve reliability and manageability of the SCSI bus and peripherals on it, while maintaining backward compatibility. The increased bandwidth supports today's complex datatypes over the World Wide Web, the corporate Intranet, or the local area network. The improved reliability and manageability are characteristics of SCSI-based RAID arrays that are migrating up into the base technology to the benefit of all SCSI users.

If different manufacturers implement different enhancements, the opportunity for confusion and frustration with no real improvement is obvious. Seeing this, a group of leaders in the SCSI storage market, including Mylex, agreed to standardize on a specific subset of SCSI-3 features, calling them collectively Ultral6O/m, in order to minimize confusion, and bringing the benefits of the new Ultra3 features to market as quickly as possible.

SCSI Background

The original motivation for a standards-based Small Computer Storage Interface (SCSI) was to satisfy the need for a more flexible, faster, command-controlled interface for hard disk drives and other computer peripherals. The standard provided for impressive new performance characteristics, connecting a host with as many as seven different devices, and data rates as fast as 5MB/sec. The ANSI published the first SCSI standards document in 1986. But even before that, work had already begun on the SCSI-2 standard.

Some of the new features incorporated into SCSI-2 include wide SCSI with data transfer at bus widths of 16 and 32-bits. SCSI-2 or "Fast SCSI" achieved 10MB/sec transfer rate with wider data paths of 16 and 32-bits, rising up to 20MB/sec and even 40MB/sec. It also allowed for an active terminator to improve storage subsystem integrity.

Several new commands were added and diagnostics capabilities were extended. Optional messages were added to negotiate wide transfers and to support command queuing up to 256 commands deep. Sense keys and sense codes were formalized and extended.

As Fig 1 shows, SCSI-3 is the sum of a number of separate standards. It is a family of standards, one of which is known as SPI-3 or the SCSI Parallel Interface-3. The SCSI implementations known as Ultra2 SCSI and Ultra3 SCSI fall into this area.

Ultra160/m: A Rational Subset

There are strong parallels between the evolution of the early SCSI standard in 198586 and today's efforts to group some of the features from Ultra3 SCSI into a subset called Ultral6O/m. The early specification was modified by industry leaders to increase the mandatory requirements for compliance with the standard. The Common Command Set evolved out of this effort by a consortium of manufacturers who were motivated to improve interoperability and user satisfaction.

The SPI-3, of which Ultra3 SCSI is a part, states that, for a product to be SCSI3 compliant, it needs to include only one of the new features defined in the specification, similar to the ambiguity around the early command set in the SCSI-i specification. Ultra3 SCSI products can include .any of the following features: double transition clocking or CRC (Cyclic Redundancy Check) or domain validation or packetization, or QAS (Quick Arbitration and Select).

Once again, a group of industry leaders have agreed to tighten the requirements for early implementations of the Ultra3 portion of the SPI-3 specification. Mylex, Fujitsu, Hewlett-Packard, IBM, LSI Logic, Quantum, and others have announced support for a subset of the Ultra3 specification called Ultra160,m SCSI. Given its critical mass of support, this subset is likely to initially become the industry's preferred implementation. The earliest implementations of Ultra160/m SCSI are coming onto the market now. General industry availability is expected late in 1999. Ulta160/m includes the first three features, double transition clocking, domain validation, and cyclical redundancy checking (Fig 2).

Double Transition Clocking

Ultra160/m SCSI doubles transfer rates to 160MB/sec by using both edges of the request/acknowledge signal to clock data. Double transition clocking enables speeds of up to 160MB/sec without increasing the interface clock rate and while using existing Ultra2 SCSI cable plants.