Ultra160 SCSI boosts performance, reliability

Key features include double-edged clocking, cyclical redundancy checking (CRC), and domain validation.

By Peter Aylaian and James Karney

Ultra160 SCSI was developed to meet the bandwidth requirements of faster hard-disk drives, networks, and the 64-bit PCI bus. The fifth generation of SCSI, Ultra160 boosts performance to four times that of Wide Ultra technology, while fully supporting previous generations of SCSI devices. IT managers and power users can take advantage of increased performance without sacrificing their investments in existing hard drives and other peripherals.

Current SCSI implementations have a maximum transfer rate of 160MBps, with next-generation designs specifying 320MBps.
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In addition to performance improvements, Ultra160 SCSI provides better reliability, manageability, and lower total cost of ownership compared to previous generations. Ultra160 SCSI increases the throughput of 80MBps (maximum) Ultra2 SCSI by sending two-rather than one-bits of data per clock cycle for a maximum data transfer rate of 160MBps. This approach, known as double-edged, or double-transition, clocking allows both the data and clock lines to run at 40MHz. Double-edged clocking is one of several advanced features of Ultra160 SCSI.

It's necessary to double the bus speed every two years to keep bandwidth available at four times the data rate of one drive. This ratio has been a traditional guideline for bus performance. However, the 4:1 ratio does not mean that it takes four drives to saturate a bus; other factors such as I/O patterns need to be considered. What the ratio does suggest, however, is that Ultra2 SCSI's 80MBps transfer rate may not be sufficient for heavily loaded systems.

Ultra160 SCSI is important as drive caches approach the 2MB range. Over the past three years, 10,000rpm Wide Ultra and Ultra2 SCSI drives have improved sustained data transfer rates from 11.65MBps to 26.34MBps. Ultra160 SCSI has boosted that rate to more than 35MBps, and SCSI is expected to deliver 48MBps next year.

Reliability and manageability

Ultra160 SCSI incorporates low-voltage-differential (LVD) technology em-ployed in existing Ultra2 SCSI products. LVD is a standard that combines the cost-reducing features of older single-ended technology with the resistance to signal noise and ground shifts of high-voltage-differential (HVD) designs.

Domain validation and cyclic redundancy checking (CRC) increase the ability to manage devices on the chain. During the boot cycle, or any time a new device is discovered, the host adapter tests for the maximum speed that the target can maintain. It works much the way modems interrogate each other and arrive at a workable communication rate. No user data is sent until a proper connection is made.

Chart shows sustainable transfer rates of current and future SCSI disk drives.
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However domain validation offers more than just setting reliable speed limits. Management software can use domain validation to monitor performance and suggest tuning of the bus for better throughput, to predict potential problems, and to alert administrators of required maintenance.

Error checking is imperative for high-speed operations. Ultra160 SCSI's CRC technology ensures data reaches its intended destination properly-with all of the data in correct form. Each data block contains extra bytes in the form of a mathematical code that lets the receiving device validate the contents. For applications like hot swapping drives in a RAID configuration, CRC is valuable insurance if a device is added or replaced while a system is online.

Ultra160 SCSI takes full advantage of all existing commands built into the SCSI standard. For example, Disconnect/ Reconnect allows SCSI devices to disengage from the host adapter's bus while performing complex tasks, allowing other devices free access to the host until it is finished. Tag Command Queuing reorders how blocks of data are moved on the bus to speed transfer.

Adding Ultra160 SCSI to a new or existing system is a simple task that just requires installing the appropriate SCSI card. Ultra160 SCSI does not require different connectors, making it easy to integrate the technology with existing systems and devices. IT managers and end users don't have to upgrade entire systems; they can retain existing products while updating selected components.

The future

The roadmap developed by the ANSI SCSI Standard Committee offers the promise of continued enhancement without the disruption of existing systems built with legacy SCSI technology. This is demonstrated in the smooth transition from single-ended SCSI-2 drives to Ultra160 SCSI.

Over the next few years, SCSI will benefit from additional improvements as maximum speeds move from 160MBps to 320MBps to 640MBps. Increases in bandwidth and improved packaging will allow up to 60 devices on a single PCI-based host adapter.

Peter Aylaian is manager of SCSI/future products at Adaptec (www.adaptec.com), and James Karney is president of CAPA.


Ultra160 and Ultra3 SCSI are compatible


By Mark Delsman
For many years, SCSI technology has enhanced performance while maintaining backward compatibility in every generation. This remains true with the latest generation of SCSI, called Ultra160 and Ultra3 SCSI.

Ultra3 SCSI devices include any or all of five features defined in the latest SCSI specification. After the specification's ratification over a year ago, several vendors subsequently defined Ultra160 SCSI as a specific implementation that includes three of the five Ultra3 SCSI features: double-edged clocking, cyclical redundancy checking (CRC), and domain validation. To date, every vendor that has announced a new generation SCSI product, whether under the Ultra3 or Ultra160 name, has included these three core features.

Several vendors use a modified Ultra160 SCSI name, such as Ultra160+, for their latest SCSI technology. These vendors have added a feature, or features, to the set of three core features in order to differentiate their products. Although a small number of SCSI vendors have chosen to name their implementation with a slight twist and may have added a feature or two, compatibility is maintained in all such variations. All flavors of Ultra160 and Ultra3 SCSI work with older generations of SCSI, and all Ultra160 and Ultra3 devices are interoperable.

The name Ultra160 SCSI is now recognized by the SCSI Trade Association to identify designs with the three key features. By establishing Ultra320 and Ultra640 as the names for future generations of SCSI technology, the association lends further credibility to the naming scheme. Each of these generations is expected to have a specific set of feature enhancements and is expected to be fully compatible with prior SCSI generations.

Mark Delsman is director of advanced technology at Adaptec, and is a SCSI Trade Association board member and secretary.

This article was originally published on March 01, 2000