Boasting 6 Gbps performance at about half the price of the equivalent
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But just how much of a challenge SAS-2 can mount against Fibre Channel's dominance remains to be seen.
There are two areas where SAS-2 and Fibre Channel compete: at the drive interface level and at a basic storage networking level. Considering drive interface first, SAS-2 looks to have the clear advantage, offering similar performance to Fibre Channel thanks to the 6 Gbps transfer rate and its price advantage. The challenge will be how this new interface integrates into the storage systems that suppliers will provide to their customers.
For new products that can be built from the ground up to leverage SAS-2 interfaces, there's little doubt that vendors will lean in that direction when building their systems. The cost advantages of SAS-2 are too great to ignore and there's limited downside in performance or reliability. It will quickly become commonplace to see storage controllers or heads that connect to the storage infrastructure via Fibre Channel or iSCSI and then connect to their physical storage via SAS-2.
For established vendors with existing systems, a move to SAS-2 enclosures may be more difficult, if not impossible. Mixing the different interconnects of Fibre Channel and SAS with the same storage controller will be difficult. Expect most of these vendors to deliver new products based on their existing controller architecture, but to leverage SAS-2 exclusively for the back-end interconnect.
When it comes to the storage network, Fibre Channel will still be the primary infrastructure even if the drives behind the controller are SAS based. The storage controller will simply connect SAS to the drives and use Fibre Channel to connect to the storage network. This will be the typical solution for most enterprises.
SAS-2: Shared storage access to virtual environments
An area where SAS as the infrastructure may be able to take hold is in the medium-sized data center market among those companies that have not invested in Fibre Channel or have only a minor investment in it. SAS-2 has the capability to connect up to 256 devices, and with a SAS expander you can add 128 SAS expanders, each supporting 256 devices. The SAS-2 standard also provides basic zoning and masking of storage to systems. This allows for very high performance shared storage in small- and medium-sized businesses (SMBs).
This is especially interesting in a server virtualization environment where the virtualization software's file system manages simultaneous access from multiple hosts. At that point, all that's needed is an infrastructure that will allow shared access. Until now, the default infrastructure for virtual environments has been Fibre Channel, with iSCSI and NFS access gaining in popularity. SAS-2 brings an alternative to that group by providing relatively simple, shared storage access to virtual environments but with the performance characteristics of today's Fibre Channel SANs. While the lack of infrastructure management tools may be an issue in large environments in the midtier, SAS-2 could prove to be an interesting alternative.
There are obviously times in any shared storage infrastructure when it's desirable for certain servers not to have any access to other servers' storage. For example, you might not want the storage from the virtualization environment to be able to access, even accidentally, the dedicated database environment. This is where Fibre Channel zoning traditionally plays so strong. In the SAS-2 world, much of the same segmentation can be achieved by leveraging secure zones that use denial of service and device access control. This lets multiple servers connect to the SAS-2 expander, but they can access only certain attached devices. Think of it as networking direct-attached storage (DAS).
Given the price points and the performance characteristics of SAS-2, it's hard to imagine a scenario where it doesn't dominate the drive and eventually the drive enclosure market -- essentially everything behind the storage controller. As an infrastructure, Fibre Channel and other protocols like iSCSI will likely be the primary method to connect to the storage controllers, but SAS as an infrastructure will clearly make some inroads.
This was first published in April 2010