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AN ANALYSIS COMPARING SCSI & ULTRA DMA

Summary
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.

Introduction
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.

Introduction To The Technical Aspects
Major innovations in parallel SCSI technology, built on a foundation of proven technology, promise to meet the needs of the marketplace both now and for the foreseeable future. This white paper describes the changes and how they will improve price-performance while protecting the customer's investment in hardware, software and training.

EIDE's Ultra DMA: Little More Than a Name Change
EIDE's latest-generation Ultra DMA hard disk drives claim to double the maximum interface burst transfer rate from 16.7 to 33 MByte/sec. However, empirical analysis reveals quite a different story. Under Windows NT?#130; Workstation 4.01, the best Ultra DMA hard drives are only 2.7% faster than the best of the previous-generation ATA DMA Mode 2 drives.

Best
Ultra DMA
Best
ATA DMA
Mode 2
Improvement
ThreadMark v2.0 5.71 MB/sec 5.56 MB/sec 2.7%
WinBench 97 v1.1 Business Disk WinMark 1,240 KB/sec 1,230 KB/sec .81%
WinBench 97 v1.1 High-End Disk WinMark 3,440 KB/sec 3,400 KB/sec 1.18%

Chart 1: In three separate benchmark tests, the best Ultra DMA hard drives outperformed the best of the previous-generation ATA DMA Mode 2 drives by just 2.7%, 0.81%, and 1.18% respectively.

Why does ATAs doubling of the interfaces maximum burst rate provide such limited performance gains?

ATA's One-Lane Highway
ATA remains a single-threaded I/O interface with the ability to execute only one I/O request at a time, which restricts total data throughput speeds. Further, the drives are the bottleneck, not allowing ATA to perform at its theoretical transfer rates. Therefore, doubling the maximum transfer  rate to 33 MByte/sec buys little as the limitation still remains the drive.

Any Ultra DMA performance benefits do not come from improvements in the interface. Instead, performance improvements come from the improved data rate of the drives themselves.

SCSI's Multi-Lane Highway
SCSI is a multi-threaded I/O interface. It can process multiple I/O requests at a time. The SCSI bus remains free until a device has information
V command, data, or status V to send on the bus, orchestrated by the SCSI host adapter. Devices with outstanding requests disconnect from the SCSI bus until they have information to send. The technology allows multiple devices to process requests simultaneously, an effective way to overlap the mechanical drive delays required to locate data. It is also an effective way to share the bus bandwidth among all connected devices. With SCSI, the throughput of all drives can be combined for data throughput of up to 80 MByte/sec.

But SCSIs multi-threading does not just provide benefits in multiple peripheral environments.  SCSIs multi-threading also allows multiple requests to be processed simultaneously by one device in a single application automatically. This queuing of commands allows the drive to re-order the requests to minimize the total time required to service them. Command queuing can be especially valuable in workstations with a single hard disk drive.

Ultra DMA does not support overlapped seeks, bus bandwidth sharing, or command queuing because of its single-threaded nature.

SCSI's Hard Disk Drives are "Best of Class"
Not only is the SCSI interface with its multi-threaded capabilities an intelligent interface, SCSI drives also have the best performing mechanics behind the interface. SCSI drives spin up to 10,000 RPM.

Best 10,000 RPM
Wide Ultra SCSI
Best
Ultra DMA
Improvement
ThreadMark 2.0 9.61 MB/sec 5.71 MB/sec 68.3%
WinBench 97 v1.1 Business Disk WinMark 1,920 KB/sec 1,240 KB/sec 54.84%
WinBench 97 v1.1 High-End Disk WinMark 5,180 KB/sec 3,440 KB/sec 50.58%

Chart 2: The best 10,000 RPM Wide Ultra SCSI hard drives outperformed the best Ultra DMA drives by a commanding 68.3%, 54.84%, and 50.58% respectively.

And the best 10,000 RPM SCSI drive is significantly faster than the best Ultra DMA drive. In absolute terms, a 10,000 RPM Wide Ultra SCSI Seagate Cheetah drive can sustain transfers at 9.61 MByte/sec.  SCSI drives spinning at up to 10,000 RPM have the most advanced mechanics, providing data fastest off the drive and across the bus.

While EIDE provides the minimum functionality and performance at the lowest cost to meet the I/O requirements of the non-demanding desktop PC user, SCSI is built for the performance I/O requirements of workstations, file servers, and sophisticated applications.

Ultra2 SCSI: Much More Than a Name Change
Ultra2 SCSI is the next generation of SCSI. It doubles the data burst rate of Ultra SCSI to 80 MByte/sec, providing greater system throughput. Also, it quadruples the maximum cable length of Ultra SCSI to 12 meters, allowing increased flexibility in adding external storage or configuring clustered servers. To date, only 7200 RPM Ultra2 drives are available and benchmarks show significant performance improvement over the best Ultra DMA drives
X 10,000 RPM drives will be available soon (Q1 of 1998) and performance will only improve.

Best 7,200 RPM Best Ultra DMA Improvement
ThreadMark v2.0 8.78 MB/sec 5.71 MB/sec 53.77%
WinBench 97 v1.1
Business Disk WinMark
1,500 KB/sec 1,240 KB/sec 20.97%
WinBench 97 v1.1
High-End Disk WinMark
4,250 KB/sec 3,440 KB/sec 23.55%

Chart 3: The best 7,200 RPM Ultra2 SCSI out performed the best Ultra DMA by an impressive 53.77%, 20.97%, and 23.55% respectively.

(This document is provided by STA - SCSI Trade Association.)


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