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Serial attached SCSI or serial ATA hard disk drives: how to choose one or the other--or both. (Tape/Disk/Optical Storage).

In looking at future storage industry needs for enterprise or desktop, which hard-disk drive is the best investment for a specific application? How can the end user save money and still have the reliability and performance required for the task? To answer these questions, a performance, feature and cost comparison between Serial Attached SCSI (SAS) and Serial ATA (SATA) hard disk drives and the considerations in choosing the optimal hard disk drive for a particular application, must be addressed.

End users are looking for ways to save money on the purchase of storage equipment and performance, capacity, reliability and scalability have to be factored into any purchase decision. Overriding future storage needs are performance and capacity, as increasingly vast amounts of data and the long distance transmission of data become the norm. With current data rates at 1.5- to 2.0Gb/s and increasing to 6.0Gb/s in the foreseeable future, performance is high on the requirements chart and cost containment is not far below.

An exciting new feature for optimized, affordable storage is combining SAS and SATA drives in a single SAS system. Each technology brings its unique innovations and functionality to the storage application as their specifications and connectivity are being made compatible due to the efforts of the SCSI Trade Association and the SATA II Working Group. To better understand the contributions made by each technology, both are explained more fully.

What is Serial Attached SCSI?

Serial Attached SCSI (SAS) carries SCSI forward to new generations of storage 110. It builds on 20-plus years of reliable SCSI technology with many enhancements added over the years. Its quality, reliability and universal compatibility will more than meet the requirements of the future enterprise storage market. Due to its SCSI forbearers, it will benefit storage management, reduce the risk of storage technology change and increase system interoperability, flexibility and scalability. SAS is point-to-point technology with expander architecture. This combination offers high performance and reliable solutions that are quickly and easily deployable in the market. Beginning in 2004, when SAS products enter the market, its effect on the enterprise storage industry is expected to be significant.

What is Serial ATA?

Serial ATA (SATA) has a solid, reliable position in the desktop industry going back to 2001 when SATA 1.0 specification was completed with a performance rate of 1.5Gb/s. Its heritage is ATA, which is firmly grounded in the internal desktop storage industry. SATA is positioned to go into the future as the serial successor to ATA, continuing on the desktop, entry-level server and entry-level networked storage systems. As a serial 110 technology, SATA is a point-to-point topology. It enables hot-plug devices, thinner and longer cables and cyclical redundancy checking (CRC) for enhanced data reliability. SATA is currently gaining enhancements from the specification called "Extensions to Serial ATA 1.0." Definition of system-level device activity SATA command queuing, extended differential voltage ranges and LED behavior are included in the extensions.

SAS and SATA Feature Comparisons

The SAS feature advantage is in data security, integrity, reliability and availability in support of enterprise mission-critical applications that must be online and running 52 weeks a year. These are online transaction applications where data availability must be constant around the clock with no data lost. SAS is full-duplex and dual-ported to SATA's half-duplex and single-port capability. SAS contains expander architecture and activity indicators. SAS will be able to scale to around 200 devices and SATA by one device. The emerging SATA II Port Multiplier specification allows up to 16 SATA devices to be connected to a single SATA host port.

SAS system architecture is being designed to support SATA protocol so that the system controller readily recognizes SATA drives along with SAS drives. SATA responds to a single initiator, while SCSI responds to multi-initiators. SAS and SATA drives can reside side by side in the same enclosure and the system controller talks to each in its own language and coordinates their activities and performance. This gives the user a high level of flexibility in configuring a storage system to provide exactly what the application requires- at the least cost.


As SATA drives become faster and gain more features, they will be suitable for entry- level enterprise applications. The advantage of SAS and SATA drives in the same system is that they will be able to operate side by side on a SAS backplane in support of more than one application. Depending on the immediate need, either the SATA drives or the SAS drives, or both, can be selected by the system. The outcome is that the total cost of the system will be less, reducing the total cost of ownership (TCO) and increasing the return on investment (ROI).

In considering the best choice of disk drive for enterprise or desktop applications between Serial Attached SCSI and Serial ATA, performance, cost, capacity, integrity, scalability and flexibility all have to be factored into a purchase decision. For the desktop and laptop markets, SATA drives are preferred because they are lower in cost and have the features and benefits that support desktop applications. For transaction processing in the enterprise requiring a high level of performance and data integrity, SAS is the preferred choice. Where entry-level enterprise applications are concerned, the decision process is more complex since both types of drives can be used, either singly or in combination.
Figure 1

SAS and SATA Disk Drive Comparison

Summary of Drive Differences

Drive Comparison Table Desktop S-ATA

Performance (Access to Data)

 Latency + Seek Time 13msec @ 7200rpm
 Command Queuing and Reordering LBA based
 Rotational Vibration Tolerance 5 to 12 rads/sec/sec
 Typical I/Os per sec/drive (no 77
 Typ I/Os per sec/drive (10 35
 rad/sec (2))
 Typ I/Os per sec/drive (20 < 7
 rad/sec (2))
 Duplex Operation Half


 Unique Code and Hardware Limited
 Variable Sector Sizes No
 Mode Page Parameter Control No
 Inquiry Data No
 Diagnostic Pages No
 Capacity Controls No


 Activity LED No
 Fault LED No


 MTBF 600K Hrs
 Duty Cycle 8 X 5
 Interactive Error Management No
 Internal Data Integrity Checks No
 Dual Port No

Drive Comparison Table Enterprise SCSI

Performance (Access to Data)

 Latency + Seek Time 5.7msec @ 15K rpm
 Command Queuing and Reordering LBA (1) and RPS (2) based
 Rotational Vibration Tolerance 21 rads/sec/sec
 Typical I/Os per sec/drive (no 319 (3)
 Typ I/Os per sec/drive (10 319 (3)
 rad/sec (2))
 Typ I/Os per sec/drive (20 310 (3)
 rad/sec (2))
 Duplex Operation Full


 Unique Code and Hardware Extensive
 Variable Sector Sizes Yes
 Mode Page Parameter Control Yes
 Inquiry Data Yes
 Diagnostic Pages Yes
 Capacity Controls Yes


 Activity LED Yes
 Fault LED Yes


 MTBF 1.2M Hrs
 Duty Cycle 24 X 7
 Interactive Error Management Yes
 Internal Data Integrity Checks IOEDC (4)
 Dual Port Yes

Drive Comparison Table

Performance (Access to Data)

 Latency + Seek Time Performance
 Command Queuing and Reordering
 Rotational Vibration Tolerance
 Typical I/Os per sec/drive (no
 Typ I/Os per sec/drive (10
 rad/sec (2))
 Typ I/Os per sec/drive (20 Rotational Vib
 rad/sec (2))
 Duplex Operation


 Unique Code and Hardware
 Variable Sector Sizes Var. Sect. Size
 Mode Page Parameter Control
 Inquiry Data
 Diagnostic Pages
 Capacity Controls


 Activity LED
 Fault LED Indicators


 MTBF Reliability
 Duty Cycle
 Interactive Error Management
 Internal Data Integrity Checks Data Integrity
 Dual Port

(1) Logical Block Address

(2) Rotational Position Sensing

(3) Queue = 16

(4) Input Output Error Detection Code

The SCSI Trade Association is based in San Francisco, Calif.
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Article Details
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Publication:Computer Technology Review
Geographic Code:1USA
Date:May 1, 2003
Previous Article:Switch-fabric requirements for next generation storage directors. (Storage Networking).
Next Article:Perpendicular recording: the next generation of magnetic recording. (Tape/Disk/Optical Storage).

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