A few weeks ago I spoke at a TechData Government Conference here in Orlando. The title of my talk was “Serial Attached SCSI: Get Started Today”. I had a pretty good crowd of ~25 people.

And every single one of them stared at me for 35 minutes like I was some new, and not particularly interesting, specimen from the Bolivian jungles. [insert cricket chirp here]

Gulp.

After the talk someone mentioned that they hadn’t really had customers asking for SAS yet. SAS drives were still too expensive, and you couldn’t buy them anyway because the OEMs were hogging the limited supply. Sure, SAS would happen one day, but not today.

I guess one of my major points was lost in my monotonic droning:

The best thing about SAS is: SATA.

If there’s one thing we can all agree on, it’s that everybody loves SATA. The drives are almost as cheap as parallel ATA, they’re faster (due mostly to Native Command Queuing and somewhat to the 3Gb/s link speed), and they have really cool, skinny, point-to-point cables.

And, drum roll please, every single SAS controller, expander, and drive backplane shipping today supports SATA!

So why put a more expensive SAS infrastructure in place just to attach SATA drives? Good question. Followed by some equally good answers. (I’ve been rehearsing.)

Performance: The easiest way to increase performance is to have more drives processing your IO requests. Every drive you add to your system should provide an additional ~200 IOs per second or ~80MB/s of throughput. With SATA controllers, you’re typically limited to one drive per port. For example, a 4-port SATA controller will support four SATA drives. That’s 800 IOPS max.

However a 4-port SAS controller, with those ports configured as a high-speed, full-duplex, 4x link running at 1.2GB/s, can connect to a daisy-chain of external enclosures, with each of those enclosures typically supporting twelve drives. It’s not uncommon for a SAS controller to support over 100 drives, or 20000 IOPS max! And as I already mentioned, SAS infrastructures support both SAS and SATA drives.

Flexibility: If you install a SAS infrastructure today, you don’t need to decide which type drives you will use today. Or tomorrow. Today you may have some archival storage needs that is best handled by large, cheap, but albeit lower reliability, SATA drives. And later, as the price of SAS drives drop, you may decide to replace your existing SCSI and FC storage with equally reliable but faster SAS storage. With a SAS infrastructure you can mix-and-match SAS and SATA drives in the same enclosure.

So, there you go. I hope I didn’t cross Toigo’s line to Marketing Droid. I admit it. I’m a closet SAS lover. Be gentle…

TT

This entry was posted on Tuesday, April 4th, 2006 and is filed under Storage Interconnects & RAID, Advisor - Tom Treadway. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.