SAS-2: Benefits and drawbacks of the newest Serial Attached SCSI version

Learn about the latest version of Serial Attached SCSI, SAS-2, including which segment of the market it holds advantages for, how it compares with Fibre Channel and SATA, and its key drawbacks.

 The latest iteration of INCITS' Serial Attached SCSI protocol, SAS-2 (or 6 Gbps SAS), brings not just higher bandwidth but also greater scalability to the data center, making it more competitive with Fibre Channel. In this podcast interview, Greg Schulz, founder and managing partner of the StorageIO Group, explains which segment of the market SAS-2 holds advantages for, how it compares with Fibre Channel and SATA, and what its key drawbacks are.

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• Internet Explorer: Right Click > Save Target As
• Firefox: Right Click > Save Link As What's the viability of SAS-2 as a competitor to Fibre Channel?

Schulz: In some ways, they coexist. When you look at Fibre Channel-based storage, what's increasingly being deployed on the back end of those FC-based arrays are SAS disk drives, whether that be [3 Gbps] SAS or [6 Gbps] SAS. From an interface perspective, you're seeing more servers being deployed with native SAS capabilities, either integrated right onto the systems themselves or shipping with adapters. So at the lower end of the market, you're starting to see more and more deployments of either SAS-attaching storage systems or SAS being used in a shared environment. At the upper level as a host-based means of getting to storage, [it's] not really a major threat. Down at the low end of the market, it's a whole different ball game, where SAS is seen as an alternative to, for example, iSCSI. [It fits in that space that] used to have parallel SCSI, where today that's being taken over by USB 2 and finding a spot for shared access between, say, two and four servers, sharing a SAS-based array. What are its key benefits of the new iteration of Serial Attached SCSI?

Schulz: The key [benefit is that] it's running at 6 Gbps, as opposed to [3 Gbps]. Certainly you get that speed. Most of the Fibre Channel disk drives that are shipping today are 4 [Gbps], with 8 [Gbps] as a connector going from the server to the storage system, but on the back side those systems are supporting either 3 [Gbps] or 6 [Gbps] SAS on an increasing basis. So you've got the speed. Here's the other thing: With [SAS-2], you have enhanced capability for SAS switching. The SAS switching occurs either on an adapter or on a server board or on a blade or within a storage system, which means the ability to support more SAS devices per port. So you have that capability of enhanced speed and enhanced connectivity. But SAS [6Gbps] also [has] enhanced interoperability and by nature when you have a SAS port, you can attach SAS devices but you can also attach SATA devices, which drives the cost per port down. So Serial Attached SCSI is cheaper than Fibre Channel?

Schulz: SAS is cheaper, in some contexts, than Fibre Channel. Here's the way to look at it: SAS is becoming competitive on a per-component cost basis to SATA. And when you think low-cost around storage, you think SATA. What's happening is, because of the mass volume that's occuring, you're seeing the SAS chip sets going on the HBAs. You're seeing them going right onto the server motherboards. You're seeing them going right onto the blades on servers without even having to go into a mezzanine card. You're seeing the SAS chip sets going onto the drives. You're seeing them going onto the storage systems. And in the course of doing that, one of the capabilities with the new SAS chip sets is to eliminate other components; so, for example, if you have a dual-port SATA disk drive, you also have to put the interposers that actually present that dual-port signal. When you do that, that's adding extra chips, which consume power and add cost to the equation. So what's happening is the new chip sets reduce the number of overall chip sets. When you think of a product in terms of material, you want to take components out while adding functionality. That helps drive the price curves down, which then leads to broader adoption. What are the key drawbacks of SAS-2?

Schulz: The drawback is, one, an awareness issue. There's a perception that SAS is DAS. Probably the big issue is that DAS is seen as only being dedicated, internal, direct-attached storage. It can be that. But you also have arrays from HP, IBM, Dell, Oracle/Sun, where you can have a SAS array that can be shared by two, four, eight or more servers. Call it a SAS SAN if you like. I think the big drawback is, one, the lack of awareness around it as an option. And you could say that on a scale-out basis, it can't scale, in terms of connectivity, like Fibre Channel. Well, yeah, you're comparing apples to oranges.[ It's the] same thing with iSCSI to Fibre Channel or SAS to iSCSI. They're apples-to-oranges-to-grapefruit-type discussions. They have their different plays, and they can coexist very, very nicely. Do any particular market segment(s) hold potential for VARs selling SAS-2 hardware?

Schulz: At the high end of the market, for the near-term future, from a server to the storage system, that's going to remain Fibre Channel, Fibre Channel over Ethernet [and] NAS, with some iSCSI. But on the back end of those storage systems, you'll see more and more SAS in terms of the disk drives that are inside those storage systems. At the lower end of the market, that's where you're seeing [Serial Attached SCSI] inside the servers for drives that are inside the servers, even in the enterprise servers that have disk drives, those servers are also now shipping with SAS drives. At the lower end, you're going to see [a continuance] from the server to the storage, as well as the drives that are in the storage systems being SAS. The third scenario is in the massive scale-out scenario, where, maybe you're building a large-scale website or large-scale hosting or large-scale cloud or even a large bulk-type storage environment. Increasingly, what I'm seeing being deployed are the smaller modular storage systems that have SAS drives as well as SAS connectivity that then attach to 1U servers, standalone servers, blade systems, rack-type systems of different types and sizes, because of the lower cost and the ability to aggregate to create very, very dense, high-performance, high-capacity solutions.

This was last published in June 2010

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