Serial Attached SCSI Hard Disk Drives – Enterprise Proven

Authors: Paul Vogt, Senior Director of Product Management
               Dale Lafferty, Director of Alliances and Partnerships
               Xyratex International

There is a technology shift taking place in enterprise storage environments that could help IT departments address some of the constraints the economy is placing on their budgets. The venerable Fibre Channel (FC) drives have been a staple in the enterprise for over a decade and their replacement by SAS drives is now in full force. Virtually all large storage system market leaders now offer enclosures with Serial Attached SCSI (SAS) disk drives in lieu of FC. While this may be good news for departmental budgets, how will the IT decision-maker feel comfortable with the long term reliability of SAS drives? There is good news about enterprise drive technology; SAS drives are enterprise proven. Market analyst firm, IDC, projects that SAS and SATA drives will replace FC drives in enterprise storage over the next five years.¹

This is not to confuse the host connectivity and requirements for Storage Area Networks (SANs) that require the attributes of FC. SAS drives have earned their place in the enterprise by providing the key features enterprise customers require:

  1. Reliability – SAS drives have been predominant in the server market since the middle of this decade and their reliability is now proven.
  2. Robust feature set – including dual ports for high availability configurations. New generation 6Gb/s SAS builds upon the existing SAS feature set with additional robust features.

  3. Performance – 6Gb/s SAS and the point-to-point architecture provides scalability advantages over FC loop architectures.
  4. Costs – by leveraging the volume of server-based storage, SAS drives capitalize on economies of scale across the storage spectrum.
  5. Future Proof – hard drive vendors have been clear that future feature, capacity, and performance improvements will be available in SAS drives.

Proven Reliability
SAS is no longer a new interface for enterprise hard drives, having begun volume shipments in 2005. Over 4 million SAS drives shipped in 2006 and 12 million in 2007, which is regarded as the crossover point between FC drives and SAS drives. In 2008 and beyond, SAS interface drives will ship in higher volumes than FC, and by late 2008 the number of SAS drives in the 3.5 inch form factor was approximately equivalent to FC – the mainstay drive for enterprise external storage. By 2012, analysts project that all enterprise drives shipped will have a SAS interface.


Figure 1: IDC’s HDD interface projection²

With standard five-year warranties on FC and SAS drives, the drive vendors are standing behind both enterprise interfaces. Figure 2 shows Xyratex’ Annualized Failure Rate (AFR) history with FC and SAS Hard Disk Drives (HDDs). Note that the SAS HDD volumes are much lower than FC throughout this sample period, therefore more variation is expected with the smaller sample size. As anticipated, both FC and SAS are well below the specified AFR. This is to be expected as the drive Head Disk Assembly (HDA) is consistent between the two drive configurations, considering the robust SAS interface.

Figure 2: FC vs. SAS field Annualized Failure Rate data

Robust Feature Set
The majority of drives shipped to date have been 3Gb/s SAS drives. In late 2008 and in 2009, 6Gb/s SAS drives began shipping. In addition to the proven reliability of the 3Gb/s SAS drives, 6Gb/s SAS adds features such as:

  • T10 Data Protection Model
  • Spread Spectrum Clocking
  • Standardized Zoning
  • Self-Configuring expanders
  • iPass™ mini-SAS cables
  • Longer cable length support and Decision Feedback Equalization (DFE)

The performance benefits of the SAS architecture are well documented – wide ports, point-to-point switched architecture, and now 6Gb/s SAS will provide performance capacity to handle not only the highest performance enterprise drives, but also the newer enterprise SAS Solid State Drives (SSDs). Several SSDs in a FC loop architecture or even a switched FC architecture can saturate a FC link. By using 6Gb/s SAS links and wide ports, scalability up to 2.4GB/s or even 4.8GB/s with a x8 link can be achieved.

One factor which helps drive down the cost of SAS HDDs is the economy of scale achieved by addressing both the internal and external storage. Market analyst firm, IDC, estimates that nearly 18 million HDDs (all interfaces – SATA, SAS and FC) were shipped into internal enterprise storage and 13 million HDDs were shipped into external storage systems in 2008.³ By having the feature set addressing both internal and external storage systems, SAS can address the entire market and drive down costs due to higher volumes. Additionally SAS interfaces are now being offered on 7200 RPM high-capacity drives, providing additional Total Available Market (TAM) and giving high-capacity drives the features and scalability benefits of SAS.

Future Proof
HDD vendors have been clear that new form factors and HDD features will be offered on SAS drives. While some exceptions exist, Small form Factor (SFF) drives will be the domain of high-performance SAS enterprise drives. These SFF drives offer many advantages over their 3.5″ counterparts such as lower power and higher performance due to shorter seek times. But performance isn’t the only SAS advantage as new SAS drives will include security features such as built-in encryption for protecting data at rest, as well as a new T10 standard, Data Integrity Field or DIF. While proprietary data check-sums have been available for years, now there is an industry standard that can check and protect data while it moves through the storage system and store this data integrity field on the hard drive. This standardization is a key benefit of SAS HDDs, allowing for vendor interoperability.

Enterprise storage vendors are now releasing higher performance storage systems with SAS HDD attach capabilities. Hitachi Data Systems became the first major storage vendor to ship a midrange array without FC drives when it rolled out an upgrade to its Adaptable Modular Storage (AMS) storage system with SAS drives.

According to Mark Peters, an analyst with Enterprise Strategy Group, the HDS move to SAS is not an absolute surprise. “But it is a pleasant surprise to see a big player be bold and not wait – as happens so often – to be pulled down the path of logic by the smaller vendors and market desire.” While HDS’ USV line has been more successful on the high end than in the midrange, Peters calls the AMS “an important part of the jigsaw puzzle that is Hitachi’s focus on the overall storage architecture.”

More vendors have now introduced SAS HDD-based systems. Based on the points in this article, IT managers can rest assured that SAS systems will provide the reliability, performance, and forward-looking feature set as well as the cost benefits attributable to SAS storage devices. SAS-based storage systems are one more way to stretch IT dollars, which are even more critical in this economic environment.

¹ IDC: Worldwide Hard Disk Drive 2009-2013 Forecast and Analysis: A Post Crisis Fundamental Shift, May 2009, Doc # 218262

² IDC: Worldwide Hard Disk Drive 2009-2013 Forecast and Analysis: A Post crisis Fundamental Shift, May 2009, Doc # 218262

³ IDC: Worldwide Hard Disk Drive 2009-2013 Forecast and Analysis: A Post crisis Fundamental Shift, May 2009, Doc # 218262

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.