6Gb/s SAS Market Update
The market for 6Gb/s SAS is in full swing. Server motherboards are configured with 6Gb/s SAS devices, or RAID controllers. SAS expanders, which allow for incremental scaling of SATA and SAS drive connects, are seeing volume deployments in systems and drive enclosures. By protecting investments in legacy middleware, SAS rapidly transformed the mainstream server market and elevated itself into the leading enterprise storage interconnect. The transition from 3Gb/s SAS to 6Gb/s SAS continued to evolve with the addition of enhanced enterprise-capabilities such as standardized zoning, and expander discovery. The addition of this increased functionality has helped to open new markets and applications, pushing SAS deeper into high-availability storage solutions.

Growth in the Enterprise
Enhancements like “Self Encrypting Drives” (SED) and “end-to-end data protection” (often referred to as T10 DIF), while not specific to SAS, are being delivered via 6Gb/s SAS components and products. In addition, multi-sourced high-capacity SAS drives are selectively challenging enterprise SATA drives, by offering architectural advantages in tiered-storage environments. Collectively, these SAS delivered features and breadth of product choices, have continued to bolster the usage of SAS.

Besides SAS’ role as the primary server storage connection, drive form-factors, product abundance, high-availability, co-existence with SATA and pricing have made SAS a solid choice for external NAS or SAN connected storage subsystems, a market segment once dominated by Fibre Channel drives. This movement into external subsystems makes SAS a leading peripheral connect for numerous “cloud” and tiered storage environments.

Expanding Performance
Obviously, performance is a key attribute for 6Gb/s SAS. Performance gains have occurred not only from doubling the transfer rate, but from protocol and architectural improvements as well. These second generation SAS products benefit from latency reductions offered by driver, firmware and hardware improvements across the spectrum of devices and components. It’s not uncommon for 6Gb/s SAS RAID controllers to support 200 drives or more, and deliver RAID sequential bandwidth performance greater than 2.5GB/s for reads and write throughput well in excess of 3.0Gb/s.

In addition, architectural enhancements, like the aforementioned expander discovery, offload the host software, and distribute the discovery function to the expander network, greatly reducing the messaging traffic necessary to configure moderate and large sized SAS deployments.

First generation SAS allowed suppliers to zone disk drives in multi-user, multi-processing systems, and in systems operating virtual machines, however, these initial schemes were considered vendor unique. 6Gb/s SAS provides a standard mechanism for drive zoning through expanders, and SAS switch designs. SAS switches are simply a specialized expander implementation for scalable SAS deployments, which improve the overall expandability, serviceability, manageability and availability of SAS based storage.

Figure 1 – SAS Switch used for sharing multiple expander based storage units between SAS connected hosts.

Combining the 6Gb/s SAS transfer rates, and ability to aggregate port bandwidth, in conjunction with a SAS switch, enables a low-cost sharable storage system, yielding impressive performance results. The effective transfer rates shown below will vary based on implementation and are influenced by the host server, the CPU, the SAS HBA/controller, the JBOD supplier, as well as the HDD, but are strongly influenced by the number of SAS links used to support these moderate build-outs.

Test Setup:
6 Servers (2 HBAs per host) connected to 4 JBOD pairs w/ 24 HDD per JBOD pair through a SAS Switch

• Multiple White-box servers w/ single Intel Xeon® 5520
• Multiple 6Gb/s SAS HBAs
• Multiple 6Gb/s SAS JBODs (4 JBOD pairs)
• One 6Gb/s SAS Switch
• IOmeter 2006 in client/server configuration
  • IO Queue Depth of 8

Results
(Performance limited by number of Shared SAS ports connected to the JBODs)

• Burst Max – 9600 MB/s
• Theoretical Max (accounting for over head) 8,400-8,600MB/s
• 7,820 MB/s Read
• 8,069MB/s Write

Figure 2 – 6Gb/s SAS Performance results in a shared storage implementation using SAS switches.

These types of shared storage systems are being used in database, data warehousing, Exchange server, pre/post video processing/production, media streaming, security, scientific analysis, and surveillance applications. This high-performance data sharing model brings true enterprise functionality to these business solutions at a very affordable price point.

Low-latency SAS enables non-volatile storage
Of course performance is also extremely important to enabling new classes of low-latency storage devices, such as SSDs. SSDs are experiencing significant adoption as a data cache and are becoming a significant link in the entire tiered- storage chain. Whether it’s database applications, data mining, data ware-housing, video production, streaming, or numerous other low-latency applications, nonvolatile memory solutions improve systems response times, under extreme workload demands.

As mentioned, a great deal of effort has gone into improving SAS’s performance and driving down protocol latency. SSD I/O performance is typically determined by the number of channels, as well as, the latencies associated with the NAND components themselves. Even so, SAS suppliers continue to invest in reducing SAS latencies even further, so as not to become a bottle neck in subsequent generations of low-latency storage solutions.

HBA controller solutions that have tuned software and firmware to take advantage of low-latency latency drives (SSDs), have boost RAID 5 I/O performance levels from 200,000 random IOPS to greater than 450,000.

With average response times plummeting to less than 200 microseconds, the ability to deliver systems achieving over 1M IOPS for 1K random read workloads, is well within the reach of today’s 6Gb/s SAS technology.

The performance graph below shows conservative estimates of what exists and what can be expected from SAS I/O performance, all intended to keep SAS at the forefront of storage architecture for HDDs, SSDs and other non-volatile storage solutions.

Figure 3- 6Gb/s SAS delivers best-of-bread performance for low-latency (SSD) storage devices

Supporting the performance demands of the industry, while maintaining the enterprise-proven benefits of SAS, will continue to keep SCSI at the forefront of new storage architectures.

Topology to spare
The demand to create and retain data, grows unabated, and while standardized zoning and discovery improve the scalability of 6Gb/s SAS, longer distances are often required to effectively attach storage devices located between storage racks, or for building additional flexibility into your physical storage deployment options.

To address this issue, an active copper cabling option, extends the distance between connected SAS products to lengths in excess of 20m. At a recent SAS plugfest event, reliability was proven for distances up to 100m with a large system build using multiple active cables, cascaded through a network of 8 separate expanders. This option to create longer spans between SAS based servers and subsystems is dramatically alternating the solution landscape to better scale with the needs of the fabric.

With these improved distances and scalability options, SAS has become a host interconnection alternative. These usage cases are less demanding on distance, but require best-in-class performance, combined with the cost and simplicity of DAS, SAS effectively competes with iSCSI and FC in numerous markets.

Moving forward….
SAS is well-entrenched in nearly every aspect of Enterprise storage. Enhanced enterprise capabilities have migrated SAS from a device connection scheme, into a critical storage interconnection fabric that serves an important role within tiered-storage solutions.

Next generation SAS devices will once again grow with the performance and scalability demands of the industry by supporting 12Gb/s transfer rates, optical connections in addition to active copper, managed cabling to improve system availability and serviceability, and multi-ported SAS implementations increasing the available system bandwidth for system builders, deploying advanced SSD solutions.

While the future direction of storage is sometimes a murky proposition, SAS will continue to enable advanced storage architectures while maintaining legacy investment protection. SCSI and SAS have survived an incalculable number of market transitions and continues to advance, even more significantly today. This resiliency, supported by viable enhancements and roadmaps, has been the essence of SCSI/SAS and promises to be continues to be so for the foreseeable future.