Key benefits of Serial Attached SCSI

SAS provides a number of advantages over parallel SCSI in the areas of performance, configuration flexibility, scalability, and reliability/availability.

By Paul Griffith

Today's predominant server storage interface for enterprise environments—SCSI—relies on parallel transmission of data streams for device-level attachment. However, parallel technology's signal skew and crosstalk, signal termination restrictions, cable and connector reflections, and device addressability are barriers to throughput performance as servers are pushed to meet advancing system and application requirements.

Serial Attached SCSI (SAS) overcomes these barriers and delivers greater speed, reliability, and scalability than parallel SCSI for bandwidth-hungry servers and storage devices. SAS addresses the high frequency, immediate random data access required for transactional applications such as online purchases and bank transactions. SAS combines the proven reliability and functionality of parallel SCSI with the performance and design advantages of serial technology to deliver the following key features and capabilities:

  • Performance—First-generation performance of up to 3Gbps (300MBps), wide ports for aggregated bandwidth, full duplex transmissions, port aggregation, advanced command queuing levels, and the rich command set of SCSI.
  • Flexibility—Physical and software compatibility with both SAS and Serial ATA (SATA) drives, and backward-compatibility with SCSI software and middleware.
  • Scalability—Broad address range to physical devices, long cables with small connectors, and connectivity to external storage systems.
  • Reliability and availability—Point-to-point connections, multi-initiator capability through expanders for simultaneous access, dual active port support, and redundant paths to targets.


First-generation SAS will deliver throughput of 3Gbps, and succeeding generations up to 12Gbps, to keep pace with increasing microprocessor speeds and application advances. In addition, SAS provides multiple point-to-point connections that enable fault-tolerant designs.

The full-duplex, point-to-point design of SAS enables simultaneously active connections among multiple initiators and high-performance SAS targets. Devices can transfer data in both directions at once to effectively double the usable bandwidth of the link rate. Narrow ports allow for a single serial link, while wide ports support multiple links, allowing the aggregation of eight SAS or SATA targets to increase total bandwidth up to 24Gbps.

Like SCSI, SAS includes advanced command queuing with 256 queue levels, providing unique intelligent data handling features like head-of-queue and out-of-order queuing. These features allow a system to reorder and reprioritize commands within the interface.

Large high-performance topologies are also made more practical by advances in cabling. SAS replaces the wide 68-pin ribbon cables for signals and the separate power cable with a single, thin four-wire SAS cable of up to 8 meters long, a compact design that improves airflow inside enclosures and simplifies hot-plug connections.


A key advantage of SAS is that its backplane design and protocol interface allow the use of both SAS and SATA drives in the same system. Though each type of drive is typically used in different applications, many enterprise users may need both kinds of drives. The ability to mix and match different drive types is a benefit for both integrators and users.

Since SATA connector signals are a subset of SAS signals, SATA devices are fully compatible with SAS controllers. The SATA Tunneling Protocol (STP), included in SAS, passes SATA commands through to SATA drives and since the SAS connector is designed as a single uniform backplane, designing a system with both drive types is relatively simple. This compatibility reduces the cost and complexity of storage designs, which increases the variety of design options.

SAS-SATA compatibility also allows systems integrators to design hybrid storage systems using common connectors and cabling. Installing or upgrading either SATA or SAS drives in the same system is simply a matter of replacing one drive type with the other: The SAS backplane connectors receive both SAS and SATA devices. However, since SATA backplanes connect only to SATA devices, backplanes with SAS connectors provide more design flexibility.

SAS's backward compatibility with previous-generation SCSI software and middleware also makes it easy to incorporate legacy components, hosts, and drives into evolving SAS topologies, eliminating new training or integration costs and the need for modifications to legacy software.


The scalability of parallel buses is limited because they share connection paths, and adding more buses with multiple initiators does little to extend this limited sharing ability. SAS uses expander hardware as a switch to simplify configuration of large external storage systems that can be easily scaled with minimal latency while preserving bandwidth for increased workloads.

This expander hardware enables highly flexible storage topologies of up to 16,256 mixed SAS/SATA drives. SAS expander hardware functions as a switch to simplify configuration of large systems that can be scaled with minimal latency while preserving bandwidth for increased workloads.

A fan-out SAS expander, for example, can be connected to as many as 128 devices, including initiators, SAS/SATA drives, and other SAS edge expanders in either narrow or wide link format. These additional edge expanders can in turn be linked to other hosts and drives, providing additional connection nodes. SAS's SCSI Management Protocol (SMP) manages the point-to-point connections in the topology.

Reliability and availability

Multiple initiators have long been used in enterprise computing to provide disk drive access to multiple hosts or host bus adapters (HBAs)—or both—and to ensure continuous data access in case one fails. However, using multiple initiators in parallel technology configurations leaves single points of failure that can block access to a device. With SAS, dual-ported devices can be used to build high-availability systems with no single points of failure.

Another way to increase fault tolerance with SAS is by using expanders to connect multiple devices to multiple initiators, which maintain concurrent operation to many devices. Commands can be sent down one link and data returned on another link in a separate connection to further increase fault tolerance.

The SAS expander feature, in combination with dual-port SAS drives and SATA drives with two-port adapters, makes it easy to design redundant systems for maximum fault tolerance. The availability of 2.5-inch dual-ported SAS drives, along with standard 3.5-inch drives, will enable fully fault-tolerant designs. This scalable and reliable connection scheme enables enterprise-level topologies that support multi-node clustering for automatic fail-over and load balancing.


Serial Attached SCSI is both the next generation in the evolution of SCSI and a significant advance in the architecture of I/O interfaces. SAS will expand on the capabilities of parallel SCSI, maintaining its best features while adding new features for improved performance, flexibility, scalability, and reliability.

Enterprise storage has entered a new era in which solutions need to address a broad variety of requirements, from the most rigorous high-availability applications to cost-sensitive bulk data storage. And SAS's compatibility with SATA will provide greater flexibility in system configuration for reduced overall cost.

Paul Griffith is strategic marketing manager at Adaptec (www.adaptec.com), which is a member of the SCSI Trade Association (www.scsita.org).

Key SAS features

I/O performance

  • 3Gbps (8 wide ports enable 24Gbps)
Performance features
  • 256 command queuing levels
  • Port aggregation for wide links
  • Full-duplex transmission
Cable length
  • 8 meters
  • Up to 16,384 devices with expanders
High reliability
  • Active dual porting on target devices
  • Multi-initiator support with simultaneous access
Fault-isolation architecture
  • Point-to-point
Device compatibility
  • SAS and SATA devices

This article was originally published on September 01, 2004