Serial attached SCSI or serial ATA hard disk drives: how to choose one or the other--or both. (Tape/Disk/Optical Storage).
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 (RV) 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 Customization Unique Code and Hardware Limited Variable Sector Sizes No Mode Page Parameter Control No Inquiry Data No Diagnostic Pages No Capacity Controls No Indicators Activity LED No Fault LED No Reliability 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) (RV) 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 Customization Unique Code and Hardware Extensive Variable Sector Sizes Yes Mode Page Parameter Control Yes Inquiry Data Yes Diagnostic Pages Yes Capacity Controls Yes Indicators Activity LED Yes Fault LED Yes Reliability 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 (RV) Typ I/Os per sec/drive (10 rad/sec (2)) Typ I/Os per sec/drive (20 Rotational Vib rad/sec (2)) Duplex Operation Customization Unique Code and Hardware Variable Sector Sizes Var. Sect. Size Mode Page Parameter Control Inquiry Data Diagnostic Pages Capacity Controls Indicators Activity LED Fault LED Indicators Reliability 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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|Publication:||Computer Technology Review|
|Date:||May 1, 2003|
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