Inside IT Storage

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SAS mythbusters: Data highways and SAS vs. SATA

With LSI’s recent announcement that the company has begun sampling 12Gb/s SAS technology to OEMs, it seemed a good time to write about SAS. Also an opportunity to clear up a couple common misconceptions.

SAS, or Serial Attached SCSI, is the current interface of choice in the enterprise for connecting drives (either HDDs or SSDs) to servers and storage arrays. Current technology is at 6Gb/s but 12Gb/s technology is just around the corner with plugfests occurring in 2012 and production hitting in 2013.

Good stuff – and double the speed too. Well…yes in a matter of speaking. And that’s the first area to clear up with misconception #1. With SAS speeds, we’re talking about bandwidth. An engineer many years ago explained it to me like this and I thought it was brilliant. He told me to think of SAS as if it was a highway system and the cars traveling on the highway represented the data from drives. When we move from generations, 6Gb/s to 12Gb/s for this example, it’s never really about how much faster the cars can go, but how much wider you make the roads so that more cars can travel on them before they are forced to slow down in “traffic”.  And no doubt about it, with the growing amounts of data that we’re expected to see based on numerous industry forecasts, we need all the highway space we can get. And 12Gb/s technology will definitely provide more lanes on the road.

Misconception #2 relates to SATA vs. SAS and really covers a number of issues about how the technologies are often judged between each other. When having discussions with people over the years, there has been the general assumption that a SAS-based system should always be faster than SATA. Afterall, SAS is the traditionally preferred enterprise interface, costs more, and therefore should be superior in all ways right? The answer though is really dependent on the application and the environment where the storage is used. SAS has a lot more intelligence onboard, and a lot of it is related to maintaining data-integrity. Well that added intelligence means more functionality, which can relate to more time needed to complete some operations. A simple example is bootup – there’s more checking going on with a SAS system at bootup that a SATA system doesn’t do. So guess which will be faster? SATA.

But what about transactional performance? Again, depends on the system. Stick an otherwise equivalent single SAS drive into a workstation (Seagate Constellation and Constellation ES drives are offered with SATA or SAS), and the various error and data integrity checks can result in slightly slower performance. But if we’re talking a group of SAS drives in a server or storage array, then we’ll start to see great performance vs. what a group of SATA drives could otherwise do. SAS allows more customization of the system and many IT pros will configure their systems through adjusting mode page settings of the drives. For example, Seagate SAS drives like Constellation allow dual-porting, can be configured for enabling or disabling write cache, and customized for power management via four modes of PowerChoice settings.

So the bottom line is that a SAS-based system will shine when there are multiple drives running in a server or storage array, as well as providing protection for mission-critical data that SATA can’t offer. But for small systems where the extra functionality isn’t required, or single-drive environments for desktops and workstations, SATA is ideal.  Each interface has its place.



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