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

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.


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

* Support for multi-port and multi-path connections

* Support for topology and link management

* Support for enclosure management

* Support for the design of new, small form factor hard-disk drives

* Improved backplane support

As a result, vendors can use the same Stock Keeping Unit (SKU) for the backplane to support both SAS and SATA drives. This reduces interoperability issues and inventory costs and simplifies product support costs.

Advantages of Point-to-Point Architecture

SAS, as a point-to-point architecture, establishes a link directly from the controller to a disk drive or through an expander-switching matrix. However, in existing parallel SCSI, only two devices can communicate at once, and as throughput needs increase, the shared-access medium can become a bandwidth bottleneck which affects scalability. Shared access topologies also are typically more complex and have arbitration schemes that are more time consuming than point-to-point architectures.

The ability to configure arrays with low-cost SATA drives or high-performance, dual-port SAS drives will simplify storage purchasing and deployment by enabling one storage array to meet a broad range of application requirements. In addition, SAS utilizes a smaller connector than parallel SCSI disks. The smaller connector is conductive to the design of high-density, small-form-factor servers, hard disk drives, and RAID arrays.

Universal Interconnect

SAS technology makes a significant step toward a universal interconnect for DAS. A SAS initiator supports four standard SCSI protocols including:

* Serial SCSI Protocol (SSP) supports full duplex connections to SAS devices. SSP maximizes throughput and minimizes latency to complement the high-performance characteristics of SAS disk drives. In a heavily queued environment, SSP supports data transfers in both directions simultaneously, so the effective throughput of a single 3-Gb/s link can increase to 6-Gb/s.

* Serial Management Protocol (SMP) supports connections to the SAS expanders within the topology to manage links between devices.

* Serial ATA Tunneling Protocol (STP) supports half duplex connections to the SATA devices attached to expanders. Expanders using STP allow SAS controllers to generate simultaneous requests to multiple SATA devices.

* SATA also supports half duplex connections in SATA devices directly attached to the SAS controller. A SATA device directly attached to a SAS controller effectively forces the SAS controller to become a high-performance SATA controller.

Cabling and Hot Swap

SAS technology continues in the SCSI tradition of allowing hot swappable disk drives. This technology further allows SATA disk drives to be hot swapped, making possible online replacement of the lower cost and lower reliability SATA drives in RAID solutions enabling deployment of SAS with SATA drives in less demanding or cost-sensitive applications. Designed from the onset to provide cost-effective hot swap support for disk drives, SAS has built-in protocol support to ensure SAS drives safely come online and avoid unnecessary loads on system power supplies.

Current parallel SCSI or ATA solutions use bulky cables that have limitations in overall length. SAS external cables operate in lengths up to eight meters while remaining thinner and easier to route. In an intentional break with SATA compatibility, SAS solutions are optimized for backplane interconnect and do not support connection to SATA cables. This departure from compatibility with SATA prevents insertion of SAS drives into SATA-only backplanes and prevents potential damage to the SATA physical components.

Advantages for Backplane Design

Today, backplane manufacturers develop separate back planes for parallel SCSI and ATA drives. Both parallel interfaces require a large number of routed signal traces, which require many interconnect layers. Each additional interconnect layer in the backplane increases cost, plus large numbers of traces introduce signal noise and conditioning problems. Conversely, the reduction in routed signal traces in serial implementations like SAS means that manufacturers are able to develop simpler back-plane designs with fewer layers and signal traces.

SAS technology leverages the backplane connector introduced by SATA and extends it by adding support for a second communication link for a dual-ported SAS drive. Another improvement is signal characteristics of the physical components to allow for longer cables with increased reliability. To minimize link errors when SATA drives are connected to SAS devices, SAS expanders and controllers are designed to compensate for the limitations of SATA drives.

A SAS backplane with expanders allows any mixture of SAS or SATA disk drives. SAS drives can fully support multi-initiator access, and SCSI Tunneling Protocol (STP) enhances SATA drives by enabling a limited form of multi-initiator access. Finally, the location of the backplane connector with respect to the disk device allows enclosures designed for 3.5-inch disk drives to also support 2.5-inch small form factor disk devices to maximize interoperability.


By taking a lead role in the development and encouraging the adoption of SAS technology, STA recognizes that customer needs are outgrowing the capabilities of parallel SCSI solutions. SAS is the culmination of efforts by OEM and Independent Hardware Vendors in the server and storage community to allow DAS storage to meet the increasing demands of customers. SAS also provides the best investment protection by:

* Allowing flexible server and storage deployment

* Providing a choice in SAS or SATA disk drives based on application demands

* Increasing storage connectivity

* Reducing the penalty for incremental capacity increases

* Improving clustering solutions

* Reducing inventory costs

SAS will have the capability of fulfilling escalating and complex enterprise storage requirements as well as providing the necessary performance and scalability to move data at gigabit speeds--speeds that meet or exceed current storage I/O performance found in ATA, SATA or SCSI systems.

SAS was developed to meet the changing demands of mainstream enterprise-class storage system customers. It will allow customers ultimate configuration, flexibility, and simplicity in their storage environments. By accommodating both low-cost bulk storage (SATA devices) and the performance and reliability demands of mission-critical applications (SAS devices), SAS minimizes customer investment while at the same time maximizing customer choice and ease of deployment. As storage requirements change, customers can easily adjust to meet those requirements, without having to replace the entire storage solution.

SAS is the solution of the future for enterprise users who will eventually need more power, easier connectivity and greater scalability as storage requirements continue to grow. Early 2004, as SAS products start entering the market, customers will appreciate the time and effort that has gone into the definition, development and standardization of this innovative and essential new SCSI technology. SAS is a storage solution that can be used indefinitely.
Table 1: Serial Attached SCSI and Serial ATA Interface Comparison

Topology SATA SAS

 Point to Point Point to Point
 with Expanders

Addressing 1 16,384

Distance (m) 1 8 **

Dual Port No Yes

Connection P to P P to P *


Speed Gb/s 1.5 1.5, 3.0

Duplex Half Full

Protocols ATA SCSI (SSP), SATA,
 STP, SMP ***


* Point to Point with Expanders providing Addressability.
** External cable. Transmission levels are higher than SATA to support
backplane use.
*** SAS supports multiple protocols. The native SCSI protocol (SSP)
support for SAS devices. SASA and STP protocols support SATA devices.
SMP protocol support for expanders.

Tonya Comer is SAS product marketing manager and Lorrie Chambers is Competitive Intelligence technical writer at Hewlett-Packard (Houston, TX)
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No portion of this article can be reproduced without the express written permission from the copyright holder.
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Article Details
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Title Annotation:Connectivity; Serial Attached SCSI
Author:Chambers, Lorrie
Publication:Computer Technology Review
Geographic Code:1USA
Date:Mar 1, 2004
Previous Article:Policies with a purpose: ensuring business continuity.
Next Article:The challenges of testing SATA and SAS: part 3: testing wide SAS.

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