'Do more for less' with 6Gb/s SAS

Posted on April 01, 2009



You're an IT manager and you've just received the disheartening news that your IT budget has been slashed; however, you've also been given the somewhat contradictory mandate to increase the performance and value of the company's storage infrastructure. In today's economic climate, this challenge is a reality faced by many IT managers looking for answers to the old paradox of doing more for less; in other words, how can you stretch your IT dollars?

From a disk/array perspective, 6Gb/s Serial Attached SCSI (SAS) will help. The second generation of SAS offers a variety of new features that will deliver higher performance while at the same time lower the total cost of ownership (TCO) for storage systems.

The next generation of SAS has many additions that will translate into benefits for IT managers, systems/storage integrators, OEMs, and end users (see table).

The most obvious advantage of 6Gb/s SAS (which comes from the T10 SAS-2 specification) is the doubling of the data transfer rate. Combined with the adoption of 6Gb/s hard disk drives (HDDs) and high-speed solid-state disk (SSD) drives on the horizon, users can expect a huge performance increase in next-generation SAS server and storage systems. And the 6Gb/s SAS interface is backward compatible with 3Gb/s and 1.5Gb/s devices.

For bandwidth-intensive, mission-critical storage applications, the performance increases can translate into cutting IT spending significantly because IT departments can deploy a single 6Gb/s SAS storage system while effectively achieving the same bandwidth as two comparable 3Gb/s SAS systems.

Zoning, another feature of the new generation of SAS, enables IT administrators to define zones to control access to different HDDs by different hosts in a domain. For example, administrators can define a security policy that only allows certain hosts to access certain HDDs containing secure information. The figure below shows an example.

Zoning is a feature that was first implemented in 3Gb/s SAS expanders; the SAS-2 specification standardized zoning such that multiple vendors' products can interoperate in a standard 6Gb/s SAS zoning fabric. The standardization of zoning in 6Gb/s SAS will ensure interoperability between multiple vendors' products; thus, IT professionals can be assured that their security policies are protecting critical information in their storage infrastructure.

Self-discovering SAS expanders are an innovation whereby each expander has the ability to discover its "neighbors" (e.g., attached expanders or HDDs) and populate its own routing tables without being managed by the host/controller. The result is a significant improvement in convergence time during initialization and discovery after topology changes (e.g., adding/removing HDDs).

Table-to-table routing no longer requires IT administrators to be aware of special ports when connecting expanders together, which eliminates the need for specialized keyed cables and keyed connectors. With table-to- table routing, SAS expanders can be connected together with any ports and, as a result, IT departments can reduce costs because they will not have to purchase and stock different keyed cables and connectors for inter-expander connections.

Another valuable addition in 6Gb/s SAS is an increase in supported cable length from 6 meters to 10 meters—a 67% improvement in distance. This increases the flexibility of cabling connectivity in the data center, as 6Gb/s SAS will be able to support longer distances between controller head units and storage arrays, as well as longer distances between cascaded storage arrays—both of which translate into improved scalability in the data center.

Improved diagnostic capability is another enhancement to 6Gb/s SAS. End users will find the enhanced diagnostic capabilities very valuable, since they can significantly decrease system failure debug time. The enhanced diagnostic capabilities will also allow IT administrators to quickly and accurately detect points of failure in the storage infrastructure, reducing the resources, time, and money spent on failure detection and troubleshooting.

In addition, 6Gb/s SAS also provides enhanced status information, such as Quality of Service (QoS) parameters and more statistical performance counters. These statistics will allow IT administrators to monitor usage of the storage infrastructure and adjust the storage resources based on the reported information.


Multiplexing is an optional feature of 6Gb/s SAS. The purpose of multiplexing is to allow for a gradual transition from 3Gb/s HDDs, as two 3Gb/s links can be interleaved onto one 6Gb/s link, thus allowing the 3Gb/s links to take advantage of the increased 6Gb/s bandwidth. The multiplexing feature shares the 6Gb/s link between two 3Gb/s links with a time division multiplexing technique.

The multiplexing functionality translates into IT cost savings because it allows more HDDs to be connected to each host/controller. For example, let's compare a 3Gb/s SAS controller and a 6Gb/s SAS controller with multiplexing support, as illustrated in the figure above. A 3Gb/s SAS controller using a 4x 3Gb/s wide port to connect to a 3Gb/s HDD array can perform data transfers to four HDDs. On the other hand, a 6Gb/s SAS controller that uses a 4x 6Gb/s wide port with multiplexing to connect to a 3Gb/s HDD array can perform data transfers to eight HDDs. So, with multiplexing, each host/controller can connect to twice as many HDDs.

Next-generation 6Gb/s SAS includes a number of valuable features that will translate into direct benefits to IT professionals and end users. Many of these features, such as increased data transfer rate, table-to-table routing, enhanced diagnostics, and multiplexing, have significant cost advantages that will allow IT departments to get more for less.

In terms of availability, component manufacturers have been shipping 6Gb/s SAS devices (controllers, expanders and HDDs) for some time; thus, 6Gb/s SAS server and external storage arrays could be available as early as the middle of this year.

IAN CHAN is a product marketing manager with PMC-Sierra. This article was written on behalf of the SCSI Trade Association (www.scsita.org).


 SAS roadmap on schedule

The next generation of Serial Attached SCSI (SAS) is quickly maturing, and SAS adopters are eagerly awaiting the availability of the first 6Gb/s SAS products. Those interested in obtaining the new 6Gb/s SAS technology may not have to wait much longer, as component manufacturers have already been shipping 6Gb/s SAS devices (controllers, expanders and HDDs) to server and external storage manufacturers for many months. The figure (RIGHT) shows the latest SAS roadmap published by the SCSI Trade Association (STA). This diagram illustrates the rollout timelines for each of the SAS technology generations.

The first official 6Gb/s SAS Plugfest was held last November at the University of New Hampshire's InterOperability Lab. This was a significant milestone for 6Gb/s SAS as it was an event where all vendors with working 6Gb/s SAS products could bring their devices for interoperability testing with all other vendors. The Plugfest was an overwhelming success as it allowed controller, expander, and HDD vendors to test interoperability between different 6Gb/s SAS devices.

The success of the Plugfest demonstrated the viability of 6Gb/s SAS, proving that 6Gb/s SAS is maturing quickly and showing that this next generation of SAS will soon be ready for mass deployment in the marketplace. It has now been several months since the 6Gb/s SAS Plugfest; therefore, component manufacturers and system integrators have had even more time to further harden their 6Gb/s SAS products.

For 3Gb/s SAS, there was approximately a 12 to 18 month gestation period after the first Plugfest before the first 3Gb/s SAS products become available to end-users. Keep in mind, however, that 3Gb/s SAS was the first generation of the SAS protocol and so it required more time for maturity. 6Gb/s SAS is a follow-on generation to 3Gb/s SAS, which builds upon the framework already laid down by its predecessor; thus, we can expect that the timeline for the 6Gb/s SAS rollout will be significantly shorter than 3Gb/s SAS.

With all the positive momentum of 6Gb/s SAS, we should expect to see the first 6Gb/s SAS servers and external storage products become available on the market for deployment by IT professionals in the middle of this year.


   6Gb/s SAS product roundup


The latest additions to Atto Technology's lineup of 6Gb/s SAS/SATA host adapters include the 16-port ExpressSAS H60F (16 internal ports) and H6F0 (16 external ports), as well as the 6Gb/s ExpressSAS H644 adapter, which has four internal and four external ports.

The ExpressSAS H60F and H6F0 have a "two-adapters-in-one" architecture, with two independent controllers delivering 16 lanes of connectivity for higher performance. All of Atto's 6Gb/s SAS adapters support up to 256 devices, x8 PCIe 2.0 connections, and incorporate the company's proprietary Advanced Data Streaming (ADS) technology for bandwidth-intensive applications.

RoC on

PMC-Sierra's PM8011 SRC 8x6G 6Gb/s SAS RAID-on-Chip (RoC) is shipping on Hewlett-Packard's Smart Array RAID controller cards (models P212, P410 and P411), which can be used in HP's ProLiant DL385 G5p and ProLiant DL580 G5 servers. The SRC RoCs combine an 8-port 6Gb/s SAS controller with an 8-lane 5Gb/s PCI Express 2.0 host interface, and include PMC-Sierra's RoCstar Contention-Free Architecture and DDR2-800 Memory Controller. Other features include a MIPS-based multi-processor subsystem and hardware XOR and Reed-Solomon engines for RAID-5 and RAID-6 acceleration.

PMC-Sierra is also shipping 6Gb/s SAS expanders and multiplexers.

LSI's latest announcements in the 6Gb/s space center mostly on design wins and partnerships. For example, LSI has announced that a variety of Taiwanese server vendors—including Acer, Asus, Gigabyte, Inventec, MSI, Quanta Computer, and Tyan—will use its 6Gb/s SAS components and/or MegaRAID technology in their next-generation servers.

LSI previously announced 6Gb/s SAS design wins with Dell, Fujitsu Siemens, IBM, Intel, NEC, Sun, and Supermicro. LSI is currently shipping 6Gb/s SAS MegaRAID adapters and 6Gb/s SAS RoC ICs.

On the disk drive front, Fujitsu Computer Products of America is shipping two lines of 6Gb/s SAS drives: the 2.5-inch, 10,000rpm, 300GB MBD2 RC and the 2.5-inch, 15,000rpm, 147GB MBE2 RC series, both of which are based on perpendicular magnetic recording (PMR) technology.

Fujitsu emphasizes low power consumption on the 6Gb/s SAS drives. For example, the MBD2 RC draws 3.4 watts in idle mode and the MBE2 RC draws 4.1 watts in idle mode (a 28% reduction in power consumption compared to previous models).

LeCroy is shipping the Sierra M6-4 protocol design and test system for 6Gb/s SAS (as well as SATA). The serial data analysis system has a native PHY implementation for lock-time and decoding of traffic, and performs functions such as protocol analysis, traffic generation, host and device emulation, and error injection for development and debugging.

For 6Gb/s SAS cabling, Quellan's Active Mini-SAS copper cables are SAS-2 compliant. The active cables use the same Mini-SAS connectors used in passive interconnects, but have provisions for an internal power supply as well as an active cable detection mechanism to avoid short circuiting. The cables support 25-meter connectivity.

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