Leveraging the benefits of Ultra160 SCSI technology

As companies everywhere face the challenge of managing ever-growing amounts of data, they are turning to technologies that can boost performance while integrating easily with their existing computing infrastructure. For years, SCSI has provided a reliable, high-performance interface for data transfer and storage. Today, the latest generation of SCSI, Ultra160 SCSI, is quickly establishing itself with breakthroughs in both performance and reliability. Ultra160 SCSI provides the advantages of higher transfer speeds, greater data integrity, easier manageability, and better overall operating efficiency. To reap the benefits of this technology, Hitachi is incorporating the leading-edge capabilities of Ultra160 SCSI into the Hitachi Ultrastar* hard disk drive family of products.

The need for higher performance interfaces
With the dramatic increase in the amount of data that today's companies must process and store, the need for affordable, reliable, and high-performance interfaces is greater than ever. To keep up with enormous growth, companies are looking for technologies that can help them expand quickly and easily by building on their existing infrastructure. One of the best interface technologies to address these requirements is SCSI. For several years, all types of companies have been leveraging SCSI technology to handle a wide range of data transfer and storage needs. The unprecedented industry-wide acceptance of SCSI technology can be largely attributed to its impressive history of providing outstanding performance gains without sacrificing compatibility from one version to the next.

A higher performing SCSI interface
After SCSI, there was Ultra SCSI, then Ultra2 SCSI--or Low Voltage Differential (LVD)--a disk drive interface that was faster and more reliable than previous SCSI standards. Ultra2 SCSI provided the significant advantages of increased bus bandwidth, faster transfer rates, greater device connectivity, and better configuration flexibility. Best of all, Ultra2 SCSI was backward compatible with earlier SCSI versions. Now, an even more powerful interface is available: Ultra3 SCSI. As ratified by the SCSI Trade Association (STA), Ultra3 SCSI refers to products that incorporate any or all of the following features of the SCSI SPI-3 standards:

• Double transition clocking
• Domain validation
• Cyclic redundancy check (CRC)
• Packetization
• Quick arbitration select (QAS)

A specific feature set of Ultra3 SCSI--known throughout the industry as Ultra160 SCSI--is currently taking SCSI to new levels of performance. Named for its superior 160 MB/sec data transfer speed, the latest generation of SCSI technology incorporates the three management features of Ultra3 SCSI that specifically affect data transfer: CRC, domain validation, and double transition clocking.

These new capabilities provide a cost-effective way to dramatically boost both device performance and reliability. Because Ultra160 SCSI is compatible with Ultra2 SCSI devices, it helps protect existing investments and ensures a smoother transition. The cables, connectors, and terminators are the same for both Ultra160 SCSI and Ultra2 SCSI. In fact, Ultra160 SCSI host controllers can support Ultra2 SCSI devices, which enables the mixing of Ultra160 and Ultra2 SCSI devices on the same bus. When Ultra160 SCSI and Ultra2 SCSI devices are mixed, each device can at operate at its full rated speed.

A wide variety of benefits
By incorporating a specific combination of Ultra3 SCSI's optimized capabilities, Ultra160 SCSI has established an effective industry standard for performance and device compatibility. The Ultra160 SCSI feature set has already been widely embraced by leading system manufacturers and SCSI drive and component suppliers such as Adaptec, Compaq, Fujitsu, and Hewlett Packard. As one of the leading companies in the storage industry, Hitachi is building on its previous commitment to SCSI technology by incorporating Ultra160 SCSI into its products, such as the Hitachi Ultrastar hard disk drive family.

• Double transition clocking enables doubled data rates without increasing clock speed.
• Domain validation optimizes interoperability of SCSI devices.
• CRC improves the reliability of data transmission and error detection.

Double transition clocking: higher data transfer speeds
Before the Ultra3 SCSI specification, data was transferred over the SCSI bus by using single transition clocking--a design that limited the maximum data transfer rate to just half of the clock speed. Double transition clocking enables Ultra160 SCSI to achieve its superior data transfer rate, a critical factor in increasing overall drive performance. Data is transferred over the SCSI bus by a double transition clock that increases the speed of the data lines. As a result, the maximum transfer rate is twice that of Ultra2 SCSI--160 MB/sec compared to 80 MB/sec. This design results in better performance, especially in environments that use extended transfer lengths or have many devices on a single bus. As an added bonus, the greater data handling capacity can also help increase reliability.

Domain validation: built-in management to reduce costs
With previous SCSI versions, the host controller determined what data transfer rate was used for each connected device. Unfortunately, there was no guarantee that the connection could actually support the negotiated data rate. With domain validation, after a transfer speed is negotiated, it is checked at the actual negotiated rate. If errors are detected, the rate is decreased until the connection is free of errors. In this way, domain validation manages the connection to help ensure drive availability, reduce installation problems, and minimize costly service calls--all of which helps lower the total cost of ownership.

CRC: increased data reliability
To improve reliability, Ultra160 SCSI leverages CRC, a proven international standard incorporated into technologies such as Fast Ethernet, FDDI, and Fibre Channel. While previous versions of SCSI used parity checking to detect transmission errors, CRC uses an additional error detection capability that is far superior for high-speed data transfer and hot-plugging situations. Because CRC verifies that all transferred data (instead of just a single byte) is received correctly, it significantly improves data reliability. CRC also provides a solid foundation for increased data transfer rates in future versions of SCSI.

Advantages for high-performance environments
Compared to other popular interfaces, Ultra160 SCSI provides a number of definite advantages. For instance, the 160 MB/sec speed is much faster than other interface speeds. Ultra160 SCSI also supports multitasking, which can significantly improve system performance, especially in workstations and servers running Windows NT**. Because SCSI technology has been so widely embraced in the marketplace, it offers a much wider array of devices from which to choose--a factor that keeps costs down and makes Ultra160 SCSI less expensive to implement. In addition, cable lengths up to 12 meters and attachment of up to 16 devices help make Ultra160 SCSI a much more flexible solution.

• Throughput-intensive digital video environments and multinode server clusters can benefit from the added performance and manageability of Ultra160 SCSI. Large disk farms with RAID configurations can also benefit from Ultra160 SCSI's high bandwidth, which might help absorb the additional workload if certain devices fail and need to be replaced.
• Internet service providers, corporate intranets, data mining, and data warehousing operations can also benefit from the ability of Ultra160 SCSI to negotiate transfer rates and guarantee connection during hot-pluggable drive replacements.
• Servers and workstations--as well as Internet, CAD/CAM, multimedia, video, digital broadcasting, and groupware applications--that require a more advanced interface to handle increased data transfer needs can also benefit from the leading-edge performance.

Figure 1. The maximum theoretical data transfer rate in MB/sec for today's commonly used interfaces. Maximum data rates are specified in the standards that relate to specific interfaces, and represent only the speed of the data without considering overhead.

Ultra160+ SCSI: the next generation of SCSI
On the heels of Ultra160 SCSI comes Ultra160+ SCSI, which features all of the capabilities of Ultra160 SCSI and includes the two additional Ultra3 SCSI features: packetization and quick arbitration select (QAS). These features are designed to enhance operating performance even further.

• Reduce protocol overhead and provide scalable performance improvements, especially at data transfer speeds of 160 MB/sec and higher
• Provide faster transfer of command and status information and the ability to transfer multiple commands and multiple threads of data per connection cycle (commands, messages, and status are all transferred at the data rate speed)
• Enable transfers to be streamed within a single connection
• Utilize a packet structure similar to Fibre Channel

Normal SCSI arbitration requires a certain amount of time during which one of the SCSI devices gains control of the SCSI bus. QAS is a new feature that provides faster arbitration than current SCSI devices by using a different protocol to determine which SCSI device gains control of the SCSI bus. This design reduces disconnect and reconnect time on the SCSI bus. As a result, devices spend less time trying to establish communications--which enables much more efficient use of SCSI devices.

Hitachi is currently implementing Ultra160 SCSI technology in the Hitachi Ultrastar family of hard disk drives. Current drives, such as the Ultrastar 36LP, 36LZX, and 72ZX already operate at 160 MB/sec, with some drives offering Ultra160+ transfer rates. For additional information about the benefits of Ultra160 SCSI technology, visit www.ultra160-scsi.com.

• More efficient command and data transfer with packetized SCSI