NetApp - Dave's Blog: SAS Disks: The Next Generation of Something

After my blog entry on disk drives, a reader pointed out that I talked lots about Fibre Channel and ATA/SATA, but I didn't say anything at all about SAS (Serial Attached SCSI).

If you recall, I argued that disk drives come in two flavors: "commodity" and "value-add". The first is high-volume and inexpensive (e.g. ATA), and the second is higher performance and higher reliability, but also more expensive with higher profits for the disk vendors (e.g. Fibre Channel).

In this context, SAS confuses me. On the one hand, drive vendors are positioning it as a successor to Serial ATA (SATA), which ought to put it in the inexpensive commodity category. On the other hand, drive vendors are touting the advantages SAS has over SATA in duty cycle, MTBF (Mean Time Between Failure) and seek time. All of this sounds like the value-add zone, where one would expect higher margins and higher prices.

This confusion makes me suspicious. Is SAS a successor to SATA (cheap commodity) or a successor to Fibre Channel (expensive value-add)? Am I really going to get that FC-style reliability and performance at SATA prices? In short, I'm afraid disk vendors will charge high-margin "value-add" prices instead of low-margin "commodity prices", and I'll end up getting screwed if I rely on SAS as a true SATA successor.

Here's how I think about it:

If SAS does go value-add, fast and reliable, competing with Fibre Channel, it's going to take time to build up the track record of quality that will allow enterprise customers to trust it with their valuable data. So again, I'm in no hurry.

But here's the good news: SAS connectors are compatible with SATA, so it's not hard to build shelves that take either type of disk. Most storage system vendors (NetApp included) seem to be designing their next generation SATA shelves so that they can also support SAS drives. (These products aren't all announced, but this is what I hear from friends of friends.)

This means that it's easy for storage system vendors to be well positioned no matter whether SAS is a commodity SATA replacement or a value-add Fibre Channel replacement. This gives me confidence that my wait-and-see attitude doesn't have a downside, even if SAS turns out to be wildly successful.

From a technical perspective, I like SAS. The connection to SAS looks a lot like SATA, and as a result you can build less expensive shelves. On the other hand, the guts of drives themselves are the same as high-end Fibre Channel drives, which means they've got good performance and reliability, and SAS has dual attach which also helps reliability. In some cases, only the firmware and connector is different between SAS and FC drives. Very cool.

So putting aside my paranoia and cynicism, I like SAS. I expect SAS to be the next generation of something. I'm just not sure what.