Transitioning to SAS technology: a comprehensive comparison between SAS and parallel storage

Computer Technology Review, March, 2004 by Tonya Comer, Lorrie Chambers

Every company, regardless of size, wants to be able to save money and have flexible mass storage equipment. When systems grow larger, making equipment choices can have a substantial impact on the bottom line. In enterprise server environments where cost, performance, benefits and architecture are key considerations, transitioning from parallel SCSI or ATA to SAS, provides the user unprecedented choices in storage solutions.

A way to keep costs lower and maintain flexibility is to transition to Serial Attached SCSI (SAS). Current server solutions are not very flexible with respect to the type of storage they deploy. Systems designers for direct-attached storage (DAS) must choose between a low-cost storage interconnect like parallel ATA, or a mainstream interconnect like parallel U320 SCSI. The SCSI Trade Association (STA) has recognized the benefit of SAS over its predecessor Ultra320 SCSI and has endorsed SAS technology as the future of mainstream SCSI.

Advantages of SAS Storage Solutions

SAS technology encompasses a set of innovations that changes the familiar parallel SCSI interconnect between a server and its storage devices. Along the way, SAS embraces the best features of other serial storage solutions and provides a roadmap for innovation and improvement now and into the future.

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SAS supports dual-port and full duplex communication between servers and disk drives. It extends the reliability and bandwidth opportunities by using expanders to create wide ports, which are multi-path connections between the server and storage solutions. It also improves manageability of the storage solution by providing Serial Management Protocol (SMP) to monitor connections between individual devices and identify points of failure or reduced performance. This technology also uses a smaller interface connector, taking advantage of small form factor SAS drives, thus enabling the development of highly space-efficient and lower power server and storage drives.

By virtue of its multi-protocol support, a SAS-based storage solution will enable customers to deploy a standardized server interface and choose the most appropriate storage for their application. Low-cost SATA drives currently being used in desktop solutions will be available for less demanding applications, while enterprise-class SAS drives will provide the reliability and performance demanded of mainstream server applications. The native SAS protocol uses the familiar SCSI command set and aspects of the Fibre Channel (FC) transport layer, which prepares SAS for low-cost bridge solutions to FC or iSCSI Storage Area Networks (SANs) in the future.

SAS solutions complement mid-range to high-end enterprise servers and workstations using internal storage, direct attached storage (DAS) or network-attached storage (NAS). Flexibility provided by SAS allows customers to tailor storage for solutions that provide mixed pools of storage within cost constraints. End users benefit from the cost reductions that SAS/Serial ATA compatibility provides to back-plane manufacturers, system OEMs and VARs. The ability to change from SAS to SATA drives without purchasing new systems simplifies the upgrade process and helps hedge end-user investments on future technology advancements.

Advantages of SAS and SATA Compatibility

By designing in compatibility with SATA drives at the connector, SAS systems allow a lower cost SATA alternative to the higher reliability and performance of SAS drives. SAS systems communicate with the existing 1.5-Gb/s SATA drives as well as the high-speed serial, signaling of the new 3-Gb/s SAS drives--with potential speeds to 12-Gb/s. SAS also enhances the point-to-point SATA topology by incorporating expanders, which are low-cost switches that allow a significant number of SATA or SAS drives to be connected to the server.

SAS and SATA compatibility enables VARs and integrators to easily configure a system for an individual customer, simply by installing the preferred disk drives. VARs can focus on matching the processing and storage needs of the customer with fewer compromises because of storage interface limitations. There is no longer a need to worry about installing the proper backplane and cables. The reduced complexity and flexibility can improve the overall customer experience.

SAS customers will be able to enjoy the following advantages:

* Increased storage connectivity

* Flexibility in server deployment

* SAS disk drive/SATA disk drive compatibility

* Total management of the storage solution

* Lowered cost of ownership

The compatibility between SAS and SATA has widespread benefits for enterprise users. One of the primary benefits of compatibility between SAS systems and SATA drives will be the ability to install a common infrastructure of cables, connectors, backplanes, cabinets and components. SAS leverages the SATA interconnect technology to have compatibility with lower cost drives.

SAS extends the SATA interconnect technology to provide:

* Increased device support and bandwidth scalability