Step up to SAS for Server Storage

Author: By Mike Micheletti, Product Manager
LeCroy Corporation

Administrators are often faced with choosing between a newer, more exciting technology or going with an existing, proven one. When it comes to server storage, the choice today is between the Serial ATA (SATA) hard drives and storage devices based on the Serial Attached SCSI interface (SAS).

SAS has become the next evolution of SCSI and significantly expands on the capabilities of its parallel predecessor. SATA has effectively replaced the ATA/Integrated Drive Electronics (IDE) standard that was used for years in desktop hard drives and CD/DVD drives. SATA drives have always been inexpensive and easy to work with, but they have also lagged behind SAS drives in terms of performance.

Serial Attached SCSI leverages years of proven SCSI functionality to deliver a storage architecture designed for server applications. SAS technology offers many features not found in SATA or parallel SCSI, such as drive addressability up to 16,000 devices per initiator. In addition to SAS’ impressive scalability, lower latency, lower command overheads and faster access times, there are other features that help SAS outperform Serial ATA in server applications:

SAS drives are built for higher duty cycles. SAS drives feature duty cycles of up to 80% which defines the percentage of time the device is rated for servicing I/O requests. Higher duty cycles allow SAS drives to stand up to more demanding data access patterns within the server subsystem. In general, SATA drives are considered desktop-class drives and are not rated above 20% duty cycles.

SATA’s command standard isn’t enterprise-class. SAS uses a method of optimizing data called command queuing, which allows the controller to retrieve data from the drive in the best possible order. On a server, where there may be dozens (if not hundreds) of concurrent data requests, command queuing can dramatically improve transaction-processing performance. The SATA implementation of command queuing does not offer the same capabilities as SAS, including support for expediting higher priority commands (immediate bit).

SAS architecture can support SATA drives. SAS backplanes and host bus adapters (HBAs) are fully compatible with Serial ATA (SATA) drives, giving unparalleled freedom to choose the optimal solution for a given application. The flexibility of using SATA drives, to provide a high capacity and low cost-per-gigabyte storage for near-line applications, is a plus for SAS.

SATA uses the CPU for managing data flow; SAS doesn’t. SATA relies on using the CPU for managing its data flow, another legacy inheritance from the ATA/IDE standard. SAS controllers offload the management of data flow to the controller’s own dedicated hardware, which means faster overall throughput. While the amount of CPU load that SATA imposes on the system is nowhere near what it used to be in the older ATA/IDE standards, there is still the overhead required to go to and from the CPU — and that’s CPU power and bus bandwidth best devoted to other things.

SAS Drives support dual porting Serial Attached SCSI’s small connector supports full dual-port connections on 2.5-inch hard disk drives, a feature previously found only on larger 3.5-inch Fibre Channel disk drives. Dual-port connections are important for mission critical applications that require redundant pathways to important data.

In short, serial ATA is best for simple, single-disk servers and desktop configurations. The SAS architecture provides a convenient way to incorporate SATA drives when performance and reliability requirements are less stringent. But for enterprise-class server applications, SAS drives will considerably outperform SATA drives due to the fact that the interface is Full Duplex rather than Half Duplex. SAS devices also have lower latency, lower command overheads, faster access times, better command queuing and offer higher duty cycles than SATA.

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