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White Papers

Easing The Migration from Parallel to Serial Storage (PDF)
This white paper, written by Adaptec, discusses the reasons for and challenges of moving from a parallal to a serial storage configuration. Readers will learn about new storage technologies with serial architectures that provide higher performance and capacity scalability and enable denser designs are emerging to streamline IT operations and reduce total storage ownership costs.

Rapid data growth among companies of all sizes is sending IT departments scrambling for cost-effective ways to store more information under tightening budgets. A new class of data - reference information - is driving much of this growth. This infrequently accessed data such as email, presentations, documents, graphics and variety of images including CAD/CAM drawings, medical X-rays and bank checks is placing especially acute cost pressures on small and midsize businesses (SMBs), the market segment accounting for the bulk of reference data growth but the one typically with the smallest IT budgets. The result: SMBs, along with their enterprise counterparts, are seeking out cost-effective alternatives to enterprise-class disk drives for this exploding class of information. They also continue to explore ways to reduce the cost of storing high-demand database information and other bandwidth-intensive transactional data.

Assessing and Comparing HP Parallel SCSI and HP Small Form Factor Enterprise Hard Disk Drives in Server Environments (PDF)

This white paper, written by HP, provides a guide for deciding whether to implement Ultra320 SCSI-class drives or small form factor enterprise drives in enterprise server environments. The paper is a technology assessment of the two classes of drives with a detailed comparison of the features, benefits, performance, scalability and cost of the two drives.

Serial Attached SCSI and Serial ATA Compatibility (PDF)
Written by Intel, this white paper is targeted to IT managers and anyone who is interested in learning more about the compatibility between Serial Attached SCSI and Serial ATA. The two technologies were defined with compatible physical layers so they can operate in the same Serial Attached SCSI system.

SCSI is the dominant disk interface technology in enterprise storage today, with ATA emerging in many entry-level systems. Some enterprise solutions utilize both standards, but the inherent differences between SCSI and ATA technologies require most of them to implement two separate storage subsystems - one for SCSI and one for ATA. As disk interconnect speeds continue to rise, existing SCSI and ATA buses are reaching their practical performance limits. New interconnect technologies are needed with roadmaps that can meet performance requirements going forward. Serial Attached SCSI and Serial ATA interfaces are emerging as solutions to these problems. Serial Attached SCSI is a serial version of SCSI, while Serial ATA is a serial version of ATA. Both technologies preserve existing investments in SCSI and ATA software applications. Serial Attached SCSI is being developed to maximize compatibility with Serial ATA, enabling a single storage subsystem to accept either Serial Attached SCSI or Serial ATA hard drives. This results in unprecedented flexibility to select the drive type that is most appropriate for a particular application as well as reducing the overall solution cost. This paper presents an overview of the benefits of Serial Attached SCSI and Serial ATA compatibility in enterprise environments.

SCSI Passive Interconnect Performance

SCSI Passive Interconnect Performance (PDF)
Paul Aloisi, Distinguished Member of the Technical Staff, Texas Instruments
Board Member of SCSI Trade Association

The SCSI Passive Interconnect Performance (PIP) standard establishes the test methods for bulk cables, cable assemblies and backplanes. It provides a method of measuring and validating the passive components in the SCSI system to ensure operation at higher speeds. The SCSI standards in the past have specified only bulk cable rules. There were no rules or guidelines for cable assemblies and backplanes.

Signal Integrity Issues to Consider When Upgrading to Ultra320 SCSI (PDF)
When upgrading an Ultra160 SCSI system to Ultra320 SCSI, there are several issues that you need to be aware of or your system will not function at the speed that Ultra320 SCSI promises. This white paper, contributed by Seagate, provides advice on what to watch for and how to make and measure adjustments to signal integrity margining.

Most developers would like to implement Ultra320 Small Computer System Interface (SCSI) by plugging in a new Ultra320 Host Bus Adapter (HBA) and new Ultra320 SCSI drives in place of the corresponding Ultra160 SCSI components. There are, however, several additional issues that need to be resolved before one can be assured that they have a reliable new system operating at twice the data rate of the old. Software, firmware and protocol issues must be addressed, resolved and verified. This paper deals with one of the basic sets of parameters that need to be determined regarding the environment in which the new equipment is operating.

SCSI Signal Modeling - The Time Has Come

SCSI-Signal-Modeling - The Time Has Come (PDF)
The job of SSM is to create the methodology for producing simulated SCSI signals that reflects a more complete set of considerations of the behavior of the components that comprise the SCSI bus.

SCSI, the Small Computer System Interface, has been a mainstay for connecting peripherals to computers and controllers for over fifteen years - quite a statement for any technology these days. SCSI (parallel SCSI) shows no signs of fading from the scene any time soon. The SCSI Trade Association (STA) is developing a roadmap for SCSI technology going out to 2012, showing confidence that this basic industry I/O architecture is far from obsolescence. The resiliency of SCSI is due to four main features...

Ultra320 SCSI: New Technology - Still SCSI

Ultra320 SCSI: New Technology - Still SCSI (PDF)
Written for the SCSI Trade Association by Adaptec, Inc.
Michael Arellano, Senior Product Manager
Contributions by: Douglas Lee, Validation Engineer, Ken Dubowski, Principal Engineer, and Arlen Young, Principal Engineer, Ph.D.

SCSI celebrates its 20th anniversary with a bang by moving to the seventh generation of the bus that introduces a maximum data transfer at a staggering 320 MB/sec. Over the course of the past two decades the protocol has evolved from an 8-bit, single-ended interface transferring data at 5 MB/sec to a 16-bit, differential interface transferring data at 160 MB/sec. For the first time the SCSI protocol has been revised in order to reduce the time spent on processing overhead, resulting in increased performance.

QAS Boosts SCSI Transmission Rates

by: Charles Gimar
Performance Analyst Storage Components Division
LSI Logic Corp. Colorado Springs, CO
Also appeared in EE Times, Nov. 3, 2000.

The growing demand for bandwidth to and from storage is putting pressure on protocols like the Small Computer Systems Interface (SCSI) point-to-point data-transmission standards. Such protocols must continually evolve to provide more data-transmission bandwidth, greater configuration options and improved management tools.

It is not sufficient to double or quadruple the bandwidth to increase performance. Additional changes must also be made to reduce nondata-overhead portions of the protocol. Quick Arbitrate and Select (QAS) is a technique, first included in the SPI-3 (Ultra160) SCSI standard, to reduce protocol overhead when devices arbitrate for and gain access to the SCSI bus. Even though QAS and Information Unit (packetized) data transfers are features of the SPI-3 standard, these features, with modification, are in the SPI-4 (Ultra320) SCSI draft standard. Systems containing Ultra320 SCSI production components implementing packetized data transfers using QAS can be expected in the latter half of 2001.

Simple Expanders Extend SCSI Bus Reach into the Future (PDF)
A white paper by David Rotman of Compaq Computer Corp. and Harry Mason of LSI Logic discussing how simple expanders (versus bridging expanders) allow for a significant increase in overall attainable SCSI bus length and SCSI domain diameter.

SCSI expanders are active interconnect components that logically connect and physically isolate segments of a SCSI domain. In addition to isolating segments, expanders allow for a significant increase in overall attainable SCSI bus length and SCSI domain diameter. SCSI expanders are available in two basic types, simple and bridging. For the purpose of this summary, we will largely concentrate on simple expanders. Simple expanders available today do not consume a SCSI ID and are capable of handling Single-ended (SE), and/or one of two modes of differential (Diff), Low Voltage Differential (LVD) or High Voltage Differential (HVD) SCSI transmission on either side.

SCSI Remains the I/O Interface of Choice for Workstations: An analysis comparing SCSI and Ultra DMA (PDF)
A White Paper prepared by Tom Martin and Andy Scholl of Adaptec, for the SCSI Trade Association

The new Ultra DMA claims to double drive performance, yet benchmark tests demonstrate a performance gain of less than five percent over older EIDE drives.
Users performing data-intensive tasks such as CAD/CAM, desktop publishing, and digital media content creation continue to use high-performance workstations as their computing workhorses. These applications test the workstation's I/O power. Now SCSI, the traditional I/O powerhouse, is facing challenges from the recently released Ultra DMA interface standard. Yet, benchmark tests on Ultra DMA drives show limited performance improvements over previous generation ATA drives, while similar benchmark tests on Ultra SCSI drives reveal significant performance improvements over Ultra DMA. Further, the performance gap widens with the next generation Ultra2 SCSI drives.

SCSI Laying the Ground Work for Performance

SCSI Laying the Ground Work for Performance (PDF)
A White Paper prepared by Paul Aloisi of Unitrode, for the SCSI Trade Association

SCSI for the present and future The Small Computer Systems Interconnect (SCSI) that has become the workhorse of the high-end system and workstations. SCSI started from a modest beginning and has continued to improve as the need for performance arises. It started from a basic interface supporting up to 5 megabytes per second, and has improved performance with wide Ultra2 to 80 megabytes per second shipping today.
The SCSI standard has been balancing the system speed requirements, system cost and a versatile interconnect to allow the widest variety of options to be connected to the system. The cost performance interface allows workstations and PC's to connect to scanners, CDRs, CDs, CD changers, tape drives, cartridge drives and hard drives with flow control to optimize system throughput.

Expanding the SCSI Bus

Expanding the SCSI Bus (PDF)
A White Paper prepared by Marc D. Brooks of Paralan, for the SCSI Trade Association

The Small Computer Systems Interface (SCSI) has been in existence for over 15 years. This parallel bus has become the most widely utilized mass storage interface in the computer industry. Its strong position in the market place permeates from the high end server applications through workstations down to the low end single user computer. The SCSI interface is projected by most industry analysts to maintain its hold on the mass storage market through the year 2005. This is due in large part to SCSI's inherent protocol flexibility, large support infrastructure, continued speed increases and the acceptance of SCSI Expanders in applications where the following needs have arisen:

Balanced LVD SCSI Drivers and Receivers

Balanced LVD SCSI Drivers and Receivers (PDF)
A White Paper prepared by Ken Stuart and Kelly Bryant of Symbios, for the SCSI Trade Association

The requirements for LVD drivers and receivers are defined in Annex A of the SCSI Parallel Interconnect-2 (SPI-2) specification and is available at:
The purpose of this article is to review these requirements and to show how a balanced LVD driver provides more system margin - and reliability -- for the system designer, system integrator, or VAR.

New Storage Interfaces

New Storage Interfaces (PDF)
A White Paper prepared by Greg Schulz of MTI, for the SCSI Trade Association

This white paper is based on the article "New Storage Interfaces" authored by Greg P. Schulz of MTI that appears in the April 1997 edition of Sys Admin Journal. This paper contains additional material not contained in the above mentioned article as well as general formatting changes.
Changes are occurring with storage technology including lower cost per megabyte, faster drives, higher bandwidths, and applications requiring larger amounts of storage and throughput. This has resulted in a storage and I/O growth explosion closing the gap between I/O, Storage, and CPU performance. Our discussion centers around storage interfaces that enable data to be stored and retrieved from storage devices (Disk Drives, RAID Arrays, Tape Drives, and Solid State Disk). Network interfaces such as Ethernet®, FDDI, Token Ring, Fast Ethernet, and ATM can also be used as storage transports for distributed or network file systems such as NFS. However, this discussion centers on technology used primarily for storage.

Parallel SCSI Grows, Shrinks and Stays the Same (PDF)
A White Paper prepared by Bill Ham of Digital Equipment Corp., for the SCSI Trade Association

This article is intended to document specific features of parallel SCSI that characterize the migrations over the past few years and into the next few years. Features that grow, those that shrink, and those that stay the same are explored.

Ultra2 SCSI - The Evolution Continues

Ultra2 SCSI - The Evolution Continues (PDF)
A White Paper Prepared by Dean Wallace of Linfinity, for the SCSI Trade Association

This white paper defines and describes the Ultra2 SCSI physical interface and discusses differential signaling. The advantages of the parallel SCSI interface and the evolution of parallel SCSI, including SCSI-1, Fast SCSI, Fast Wide SCSI, Ultra SCSI and Wide Ultra SCSI, are discussed.

SSA and Fibre Channel Myths and Realities

SSA and Fibre Channel Myths and Realities (PDF)
A White Paper prepared by Greg Schulz of MTI, for the SCSI Trade Association

This paper reviews some myths about IBM® Serial Storage Architecture (SSA), Fibre Channel (FC) and traditional Small Computer System Interface (SCSI) technologies. Some will argue that parallel SCSI is at or near the end of its life cycle while SSA is a newly established architecture with Fibre Channel still evolving. SSA and Fibre Channel, including Fibre Channel-Arbitrated Loop (FC-AL), share many common characteristics:
  • Both are serial architectures sending a stream of bits over a communication path
  • Both use fiber optics to digitally transmit high-speed serial bit streams
  • Both support more devices over longer distances than parallel SCSI
  • Both are open standards although SSA is perceived as an IBM proprietary protocol
  • Both provide improved bandwidth compared to traditional parallel SCSI with FC being
  • faster
The following are some myths and realities involving SSA, Fibre Channel and parallel SCSI. Similar to parallel SCSI, which evolved from SCSI-1 to SCSI-2 and now SCSI-3, Fibre Channel is deliverable today while still evolving. SSA has also evolved into a stable product with limited support from the industry.

Advances In SCSI Parallel Interface Technology (PDF)
A White Paper prepared by the SCSI Trade Association

The Interface Dilemma:
The computer's insatiable appetite for higher I/O performance is now further accentuated by the Internet, network servers, multi-media, video-on-demand and other demanding applications. More users need more data from more devices at higher speeds, while expecting greater levels of reliability, availability and serviceability at lower costs.