SAS Usage in External Storage Systems

Bob Brencic, Senior Director of Product Marketing
              Michelle Ng, Director of Product Marketing
              Emulex Embedded Storage Products

Serial Attached SCSI’s (SAS) use in disk drives and external storage systems is one of the hottest topics in the data storage industry today. Already gaining volume momentum in server-based applications as it replaces its SCSI technology predecessor, SAS is now starting to migrate into entry-level external storage systems as well. However, applying SAS to external storage systems presents a different set of challenges than those confronted within servers, because external storage system applications demand uncompromising performance and reliability. This article discusses how SAS will be deployed and used in external storage systems.

Why SAS is Being Deployed in External Storage Systems
The success of SAS in server-based applications is clearly driving its emergence into the back-end embedded storage network of external storage systems. Many industry experts expect that as SAS sales volumes from servers combine with SAS shipments in external storage applications, the total cost of SAS-based solutions will be less than other alternatives. Many also look to the size of the investment focused on SAS solutions today, expecting it to help spur new features and capabilities that currently are unavailable. In the long run, these new capabilities could overshadow today’s perceived primary SAS benefits of native Serial ATA (SATA) disk drive attachment and delivery of 2.5-inch small form factor (SFF) disk drives.

How SAS Will Be Deployed in External Storage Systems
Today, SAS is being deployed in the back-end embedded storage network of entry-level external storage systems (selling below $15,000) that have traditionally used SCSI technology and which provide attachment up to a maximum of 100 disk drives. Most of these systems will be initially deployed in fairly small configurations with up to 30 disk drives. Several companies have recently announced entry-level systems primarily focused on connecting via direct attached storage (DAS) or via small Fibre Channel Storage Area Networks (SANs) for small to medium-sized businesses. This is an excellent area for a newer disk drive technology to prove itself, as the performance and reliability demands are not as extreme as those imposed by the larger mid-range systems. The cost benefits that SAS permits can be most valuable in the small-to-medium sized market.

SAS is expected to start to be delivered into the back-end of the lower parts of mid-range storage systems in 2008. These will still be modest systems (costing below $50,000) and will build upon the experiences learned in initial lower-end deployments. Given that this is an area that has been dominated by Fibre Channel technology, new issues and challenges are sure to arise as the expectations for performance and reliability are high and moving backwards is untenable. By 2009/2010 the number of disk drives attached within a storage system will increase into the 500 disk drive range, and will demand significantly higher levels of performance and reliability than was required in the entry-level deployments seen today. This will place new incremental demands and requirements on SAS components and technology not available in today’s product offerings.

Only when SAS is completely field-proven and matured in the mid-range systems can it be expected to migrate into the back-end of high-end monolithic class systems, where the ultimate tests in performance and reliability will occur. This category is heavily served by proven, mature Fibre Channel technology. A migration to SAS will not likely occur until 2011/2012 after SAS proves it can deliver the necessary levels of robustness demanded by these systems.

SAS Implementations in External Storage Systems
Today’s external storage systems use an embedded storage network to connect the RAID controllers to all of the disk drives within the back-end of an external storage system. This back-end embedded storage network is finely tuned and designed specifically for connecting and passing data between RAID controllers and disk drives located in disk drive enclosures. The embedded storage network also extends out the front of the external storage system to connect directly to servers for a DAS application or to an external Fibre Channel SAN fabric. This front-end interface is and will continue to be predominantly Fibre Channel SAN connections.

Implementing the embedded storage network requires a Fibre Channel I/O Controller (IOC) at the front-end of the storage system as well as a back-end IOC that supports Fibre Channel for Fibre Channel-based systems, or SAS and SATA for SAS-based systems. The back-end SAS IOC must be able to scale and deliver a very high level of performance as the number of disk drives to which it connects increases via SAS expanders or switches. Also, while not a requirement, an IOC which combines both of these functions and is implemented as a single-chip solution reduces the overall cost and complexity of the design, as well as reducing the power and cooling required to run the array — a factor which is becoming increasingly important in today’s dense data centers.

While SAS supports the traditional cascade topologies found in many of today’s back-end designs, it also supports tree topologies allowing greater capacity with minimum congestion and greater performance. In the cascade topology (shown in Figure 1), the maximum bandwidth is constrained by the cascade links, but in the tree topology the bandwidth increases with each inter-expander link. The tree topology also provides uniformity of access to all drives in the tree compared to a cascade where access is a function of the depth.


Figure 1 – External Storage Back-End SAS Topologies


New Architecture for SAS

A new family of I/O Controllers entered the market in the fall of 2006. This new architecture provides a strong foundation onto which storage system providers can build SAS-based external storage systems. The family is comprised of a wide range of processors and developer tools designed for storage applications requiring low-power consumption and high-performance processing.

One example of the architecture used to implement SAS storage systems is a single-chip Fibre Channel and SAS/SATA I/O Controller with an internal processor built around the core. This new IOC gives storage system providers the lowest-cost solution because it integrates three typically separate components into a single chip; the front-end Fibre Channel IOC, the back-end SAS IOC, and the microprocessor that independently runs RAID applications. In addition, the IOP is perfectly suited for the demands of external storage systems because it provides capabilities that enhance system performance and reliability. For example, it contains two processors – one for Fibre Channel and SAS processing, and one specifically for the RAID application. This allows maximum performance to be delivered to its 4Gb/s Fibre Channel and 3Gb/s SAS ports while simultaneously running a RAID application.

SAS technology is driving significant changes to data storage, within not only servers, but also the back-end of external storage systems. As SAS is deployed in increasingly larger and more complex external storage systems, its core technology will be enhanced to provide compelling new solutions for these systems.

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