SAS, The Best Choice for Deploying Solid State Disks (SSDs) – LSI

Author: Harry Mason, President, SCSI Trade Association
Director of Industry Marketing, LSI Corp.

stock1_2010.jpgSSDs are low-latency storage devices that are finding their way into workstations, servers, and networked storage devices. Many of the uses for these devices are found in system-level caching environments, however recent cost improvements have made SSDs attractive for accelerating applications such as database searches.

SSDs are low-latency storage devices that are finding their way into workstations, servers, and networked storage devices. Many of the uses for these devices are found in system-level caching environments, however recent cost improvements have made SSDs attractive for accelerating applications such as database searches.

SAS, and its counterpart, SATA, are playing a very significant role in how SSDs are deployed in this emerging market.

Low Qualification Risks and Serviceable Software Model
First of all, there is a rich resource of proven software that minimizes the integration risk for SAS-based storage devices. As proven with hard disk drives (HDDs), SAS has successfully demonstrated the ability to co-exist with SATA devices, thus allowing drives to be intermixed over the same physical interface. Leveraging this well-entrenched logical interface improves the overall system qualification time using standards that already exist, and allows capabilities like secure drives and data integrity, to be conveniently ported to this new storage medium.

Surprising to many, the SAS software stack, which inherently contains the ubiquitous SCSI abstraction layer, is very efficient and has continued to improve with time. The advent of low-latency solid state storage devices has brought about dramatic performance improvements with SAS, which are still accelerating. (See Figure 1). Even today, these performance levels dramatically exceed the capability of the system to take advantage of these increases in bandwidth and I/O improvements, as a result of being limited by application and system architectures. Expect even greater SAS protocol and performance enhancements in the weeks and months to come.

System Components

  • One Eight-Core processor / PCI-Express 2.0
  • 6Gb/s SAS Controller
  • 24 Drives – (2.5″ 6Gb/s SAS 15K RPM, JBOD)
  • Vs. 4 Drives – (3Gb/s SATA SSD – RAID 0)
  • Windows 2003 64-bit


  • Iometer Benchmark
  • 4KB random workloads
  • (Typical of messaging, databases, web services)
  • 24 HDD vs. 4 SSD
  • IOPs; 11K vs. 68K (random write)


  • Performance Test Platform
  • Proposal – November 24, 2009
Figure 1: SAS performance improvements over time

While the market landscape is ever-changing, recent benchmarks have shown that well-designed SAS controllers deliver greater performance than motherboard SATA controllers or even 6Gb/s SATA controllers when used in conjunction with high- performing 6Gb/s SATA SSDs.

Pay As You Go
One of the strengths of SAS is its ability to scale storage incrementally as the needs of the end-user grow. The SAS architecture does not impose an “architecture tax” on the system, and allows the user to “pay as you go.” This model for serving the user has multiple inherent advantages. First, the ability to intermix near-line drives, with enterprise drives along with low-latency SSD drives, provides an unmatched scalability aspect with SAS.

Furthermore, SSDs are difficult to scale in some environments, given the slot and power limitations of PCIe. The SAS model of providing distributed power, insures that power requirements are optimally provisioned for the mandatory server resources, while promoting the ability to scale performance and capacity as necessary. Here again, there is no “architecture tax” to support this ability to scale system power as required.

Robust System Infrastructure
SAS also provides a convenient servicing model for replacing failed devices. Hot- pluggable drives may be readily inserted and removed without affecting system operation, while standardized industry form-factors facilitate the multi-sourcing demands of these robust storage solutions. These numerous infrastructure advantages, including the readily sourced cabling, connectors, testers and drive enclosures, provide a convenient servicing and enterprise-proven management framework.

The “Logical” Advantage
The logical nature of SAS hides the underlying technology from the operating system, and allows complex storage devices to be readily supported with existing storage management software and middleware.

This abstraction layer also promotes multi-sourcing from like vendors even when the underlying technology varies significantly between vendors. This ability to insulate storage software from the underlying storage medium, has been a hallmark of the SCSI command set, and explains much of its market-based success. SAS allows new technologies, such as SSD, to participate in these large, well established markets without requiring lengthy and expensive infrastructure development programs.

Logically, capabilities like Secure Encrypted Disk (SED) or the T10 Data Protection Model (DPM, DII, DIX) can be leveraged into the NAND flash market almost seamlessly. The traditional demarcation of software and firmware stacks that span the OS to the controller can be preserved, thereby lowering the software maintenance and development costs. In addition, it also preserves and lowers the cost of well-understood compatibility testing.

Being neutral on the technology makes SAS nearly impervious to market shifts to new technologies like 3-D and phase-change-gain relevance, and makes it easy to preserve features developed for HDDs, which will be critical to using solid state devices within the enterprise.

SAS Simplfies SSDs
SAS is a proven performance-oriented interface that allows for scaling and tiering of multiple storage types. As low-latency devices have demanded more performance, SAS has responded with greater levels of efficiency with additional performance innovations underway. The robust SAS infrastructure facilitates all aspects of configuring complex storage systems from testing through cabling, connecting, servicing and sourcing. The logical abstraction layer provided through the SCSI command set, isolates the system from inevitable perturbations in the underlying technologies, thereby lowering integration risk and greatly improving time-to-market advantages. Preserving these traditional software layers lowers the costs of integration testing, improves compatibility testing, leverages existing OS interfaces and middleware, lowers the overall cost of software maintenance and facilitates multi-sourcing. SAS is a proven-performance storage interface that simplifies the integration of any new storage technologies, especially SSDs.

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