SAS: Improving Performance, Lowering Cost

Bob Griswold, Senior Technical Program Manager Lead, Core Operating System Division, Storage Platform,

Finally, the most pervasive enterprise technology has come to a crossroads. Parallel SCSI was the heart of the datacenter – used as the standard hard drive connection and communication scheme for server and storage environments. Companies have been developing and utilizing SCSI for over 20 years as the mainstay for enterprise storage, but the technology is now transitioning to Serial Attached SCSI (SAS).

In the enterprise, where Microsoft has its biggest growth opportunities, SAS has crossed over to being pervasive in the industry. This is a trajectory point where SAS is now taking over from parallel SCSI. So any question that any of your customers may have as to what technology they should choose for direct-attached storage in order to be able to use it with Windows in their enterprise, SAS is obviously the clear choice. SAS builds on 25-plus years of SCSI industry influence and growth, and is the logical next choice for most customers.

SAS is a high-performance solution that leverages proven SCSI functionality, and builds on the enterprise expertise of multiple chip, board, drive, subsystem and server manufacturers throughout the industry. Only SAS combines the proven reliability and functionality of SCSI with the performance and design power of serial technology by delivering the following features and capabilities:

  • Performance – 3Gb/s (300 megabytes/second), wide ports for aggregated bandwidth, full duplex, port aggregation, advanced command queuing and the rich command feature set of SCSI
  • Scalability – Broad address range to physical devices, long cables with small connectors, and connectivity to external storage systems
  • Flexibility – Physical and software compatibility with SAS and SATA drives, and backward compatibility with SCSI software and middleware

SAS today delivers throughput of 3Gb/s and succeeding generations will go up to 12Gb/s to keep pace with technology and application advances. In addition, SAS provides multiple point-to-point connections that enable fault-tolerant designs.

The full-duplex, point-to-point nature of SAS enables simultaneously active connections among multiple initiators and high-performance SAS targets. 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 available bandwidth to 24Gb/s.

By combining multiple PHYs together, wide ports are created that can support the significant bandwidth requirements of large SAS topologies. Like SCSI, SAS includes advanced command queuing with 256 queue levels, providing unique intelligent data handling features such as head-of-queue and out-of-order queuing. These queuing features are critical to enterprise applications, because they allow a system to reorder and reprioritize commands within the interface.

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, 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 256 mixed SAS and SATA drives. SAS expander hardware functions as a switch point to simplify configuration of large systems that can be scaled with minimal latency while preserving bandwidth for increased workloads.

A key advantage of SAS is that its backplane design and protocol interface allows the use of both SAS and SATA drives in the same system. Though each drive type is typically used in different applications, most enterprise users have the need for both drive types. The ability to mix and match these drives is a powerful benefit for designers and users.

SATA drives will be primarily designed for cost-effective bulk storage. To achieve economies of scale, SATA drives feature lower spindle speeds (typically 7,200 rpm), lower mean-time-between-failure rates, and lower cost. Consequently, they tend to be applied where transaction rates are low and data availability is not critical.

SAS drives, on the other hand, are built for high-performance, high-availability use. SAS drives will operate at higher spindle speeds (up to 15,000 rpm) with compensation for rotational vibration to assure data integrity, and are built for higher reliability. SAS drives will be used in environments where data volumes are high and data availability is essential.

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

SAS and SATA compatibility also allows system builders 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 should use SAS connectors to provide the greatest system design flexibility.

SAS and Windows
The SCSI stack, which has been an integral part of Windows for years, doesn’t have to be changed. It works with SAS. That means you can keep going forward with SAS. SAS’ backward compatibility with previous generation SCSI software and middleware also makes it easy to incorporate legacy components-hosts and drives-into evolving SAS topologies. This eliminates new training or integration costs and the need for modifications to legacy software.

Over the past three years, Serial Attached SCSI (SAS) has been unveiled as the next evolution of the SCSI standard – featuring increased performance, scalability and reliability, while maintaining ease-of-use and the SCSI feature set that has made SCSI the de facto standard in enterprise computing environments.

SAS delivers the high performance, scalability, and flexibility required for bandwidth-hungry mainstream servers and enterprise storage. SAS lends itself to the high-frequency,
immediate random data access required for transactional data applications such as online purchases and bank transactions, and provides the performance and security required for
mission-critical applications which demand data redundancy.

Bob Griswold is the Lead Senior Technical Program Manager in the Core Operating System Division, Storage Platform at Microsoft.

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