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Ultra2 SCSI (LVD): The Low-Risk, High-Performance Solution

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SCSI Advances

SCSI technology is continually evolving, providing more bus bandwidth, increases in packaging and density, and increases in configuration flexibility. One of the more recent developments is Ultra2 SCSI (LVD), which has a low-voltage differential (LVD) interface and transfer rates of up to 80 megabytes per second (MB/sec).

Ultra2 SCSI (LVD): The Low-Risk, High-Performance Solution

As corporations commit more critical information to computers, the requirements for more storage capacity and faster data movement are increasing at a startling rate. Simultaneously, the Internet has seen dramatic increases in traffic. These factors require that the interface (bus) between computers and disk drives transfer unprecedented volumes of data at high speeds.

Interface performance is a critical factor in optimizing Internet and computer system enhancements. Some factors driving interface performance are faster drives, higher-speed microprocessors, greater amounts of memory, and more data-intensive applications.

Ultra2 SCSI (LVD), the latest implementation of the storage industry's dominant, proven interoperability technology, provides the capacity and performance demanded by today's data-intensive applications. It is easily integrated with current hardware and firmware. Compared with other alternatives available today, Ultra2 SCSI (LVD) prevents system performance from becoming I/O limited as drive throughputs increase. Ultra2 SCSI (LVD) integration performs faster drive buffer-to-host transfers, freeing the bus to accommodate more devices. The enhanced Ultra2 SCSI (LVD) is fully backward compatible, and uses the same physical environment as its predecessor. More important, it can be integrated without having to modify or change operating systems, thereby protecting the user's investment in operating system software and drivers. The literally millions of lines of SCSI software code already written can be used immediately with the next-generation Ultra2 SCSI (LVD) interface.

Ultra2 SCSI (LVD) is well positioned for the future because it offers forward compatibility and significant room for growth to support faster microprocessors and drives. Other benefits include minimal SCSI software or protocol changes, with the support of 15 years of backward compatibility experience with SCSI. Currently, Ultra2 SCSI (LVD) is capable of achieving transfer rates of up to 80 MB/sec. Ultra2 SCSI (LVD) is low cost, easy to integrate, and an optimal solution for improved performance and increased device connectivity. It is available and fully implementable now.

The SCSI Evolution

SCSI data rates have increased over time, doubling about every five years.

SCSI Term Bus Width (bits) Speed (MB/sec)

SCSI-1 8 5

Fast SCSI 8 10

Fast Wide SCSI 16 20

Ultra SCSI 8 20

Wide Ultra SCSI 16 40

Ultra2 SCSI (LVD) 8 40

Wide Ultra2 SCSI (LVD) 16 80

Wide Ultra3 SCSI (LVD) 16 160

The Ultra2 SCSI (LVD) Advantage

Major innovations in parallel SCSI technology, built on a foundation of proven technology, promise to meet the needs of the marketplace now and for the foreseeable future. Implementation and configuration of Ultra2 SCSI (LVD) is well understood, and engineering skills are available to make the transition a low-risk one. The increase in bus speed improves system performance for I/O-intensive applications. Improved noise immunity leads to better signal integrity and data reliability. In addition, the Ultra2 SCSI (LVD) low-power feature and new smaller connectors enable four rather than two SCSI buses on a single host adapter card.

Quantum currently has two high-performance product lines available with multimode LVD and single-ended Ultra2 SCSI (LVD) and Ultra SCSI interfaces. The Quantum Viking II product family provides the storage and performance needed to meet the escalating requirements of PC servers, storage subsystems, arrays, workstations, and performance PCs. Quantum also offers the Atlas™ III product family, which enhances the performance of enterprise systems, storage subsystems, arrays, and departmental servers.

Ultra2 SCSI (LVD) and Other Competitive Interfaces

The goal of any peripheral interface is to provide reliable connectivity and avoid being a bottleneck. Just as the Internet has seen dramatic increases in traffic, systems that serve the Internet must deliver unprecedented volumes of data. Internet as well as other applications impact hard disk drives in two ways. First, the drives must be faster, which in turn puts strain on the interface to keep up. Second, the sheer volume of data being stored in some systems requires a much larger number of drives than ever before. These drives are all being connected with the SCSI interface using a large number of SCSI buses. Clearly this interface has come a long way, but the market is demanding a giant step forward.

Also, since new generations of higher-speed microprocessors are emerging and disk densities are increasing, hard disk drive manufacturers are rolling out drives with faster read channels and higher sustained data transfer rates. Improving internal data rates, however, creates a need for faster external (buffer-to-host) data transfer rates.

The relatively new transfer protocol pioneered by Intel and Quantum-Ultra DMA-provides a 33 MB/sec bus rate between the disk drive buffer and system memory. While this rate is double that of the previous bus protocol, Fast ATA, it is still less than half the rate of Ultra2 SCSI (LVD).

Serial interfaces have been proposed; however, the transition to a new serial drive interface won't be fast, smooth, or inexpensive. Adopting a serial interface translates into considerable work and costs for systems manufacturers. System OEMs need a high I/O throughput solution now, not two or three years from now.

Fortunately such a solution has arrived in the form of the new Ultra2 SCSI (LVD) interface. Ultra2 SCSI (LVD) represents an enhancement of the current parallel Ultra SCSI interface standard that doubles SCSI drive data transfer rates while maintaining backward compatibility.

Ultra2 SCSI (LVD) Performance

Speed-40 MB/sec (8-bit), 80 MB/sec (16-bit)

Cable Length-up to 12 meters

Number of Devices-16 maximum

Signaling-Low-Voltage Differential (LVD). Offers better reliability, backward compatibility through automatic mode sensing of single-ended or differential bus configuration, common-mode noise immunity, reduced EMI, low power requirements, insensitivity to ground shift, extended cable lengths, and greater system margins.

Drive Throughput vs. I/O Bandwidth

Ultra2 SCSI (LVD) provides sufficient bus bandwidth to stay ahead of the throughput of a multi-drive system, as shown in the figure below. A system implementing Ultra2 SCSI (LVD) with a bus rate of 80 MB/sec yields a comfortable bus margin over the throughput of next-generation drives.

Hot Pluggability

Hot pluggability is the ability to insert or remove a drive from a system with minimal side effects. Levels of hot pluggability include those listed in the SCSI Parallel Interface (SPI-2) document which may be found at The higher the hot pluggability level a drive achieves, the less disruption the drive causes in a RAID system when it is inserted or removed. If the drive can be inserted or removed only when the RAID system is turned off, the drive is not considered hot pluggable.

At present, the SPI-2 document does not require nor prohibit hot pluggability for Ultra2 SCSI (LVD) drives. Some preliminary testing indicates that Quantum Viking II drives can be "hot inserted" into a SCSI bus (drive data cable connected with drive power off, followed by turning on drive power) without disrupting the system. Furthermore, the system responds with the appropriate SCSI processes to recognize the new drive and renegotiate modes (from single-ended to Ultra2 SCSI (LVD) mode or from Ultra2 SCSI (LVD) to single-ended mode).

Ultra2 SCSI (LVD) testing is ongoing. Testing will be better controlled as new JBOD (Just a Bunch of Drives) enclosures are developed that include LVD backplanes. An evolutionary development and testing process will take place as the new JBODs become available. The steps in this evolution are:

1. Development of a multidrive LVD JBOD

2. Development of a multidrive RAID-compatible LVD JBOD

When a stable multidrive RAID-compatible LVD JBOD is developed, the process of testing and verifying the hot pluggability of Ultra2 SCSI (LVD) drives can continue in earnest using reliable, repeatable, and predictable drive insertion and removal mechanisms.

SCSI for the Next Millennium

Over the coming years, SCSI technology will continue to evolve, providing the following advances:

  • Increased peak in bus bandwidth

  • A two-fold increase in packaging and bandwidth density on systems, allowing up

to 60 drives on a single PCI slot

  • Increased flexibility in systems design and configuration

  • Increased device addressability and configuration flexibility through bridge


  • Increased allowable bus length beyond 25 meters

  • Improvements in all levels of technology including bus length, peak I/O bandwidth,

cables, connectors, and development of new components for systems integration



Ultra2 SCSI (LVD) is ideal for increasing cable distances and the number of connected devices while achieving up to 80 MB/sec transfer rates. Quantum's Viking II and Atlas III families of products support Fast, Ultra, and Ultra2 SCSI (LVD) transfer rates. The advantage of the 15-year backward capability with SCSI allows even the oldest SCSI devices to work easily with the latest hard disk drives incorporating the Ultra2 SCSI (LVD) interface. Based on a mature and industry-proven hardware and software architecture, Ultra2 SCSI (LVD) provides a high-performance, low-risk reliable upgrade, as well as a higher bandwidth for data-intensive applications.

See Also