SAS, SCSI, or SATA: Will Server Storage Solutions End Up On Your Desk?

Small Computer System Interface, or SCSI (pronounced scuzzy) has been around for a long time. Most home users’ recollection of the term will date back to hard drives before the IDE/ATA standard (described in the previous article) took over. On the server side though, SCSI has been constantly updated and improved, remaining a popular choice.

While the cost and complexity of SCSI are undeniable, neither is its speed. The last specification for SCSI, Ultra-320, has offered 320 MB/s speeds since 2002. The reason the last spec is from ’02 (a later Ultra-640 spec never got off the ground) and there are no plans for a new one is that the SCSI Trade Association has been moving towards SAS, or Serial Attached SCSI.

SAS launched in 2005, at a speed comparable with the technology it was meant to replace (300 MB/s vs the 320 MB/s mentioned above). Actually the difference wasn’t that big, since SAS uses full duplex and can move data in both directions at once. Also, SAS ports can be aggregated to make “wide ports,” allowing four to be combined for a 12Gb/s transfer speed. There is a 6Gb/s SAS specification under testing that will become available this year, which will raise speeds even further. And SAS is cheaper than SCSI.

SAS is also more flexible. Getting more than 8 or 16 SCSI devices to work together can be complicated. SAS can support thousands of drives. Physical management is easier as well. Desktop storage connections could no longer get faster because (among other things) the parallel technology would require an even wider cable, and the cable was already ridiculously wide. SATA, a serial technology, uses far smaller cables. SCSI to SAS is also a move from parallel to serial, and comes with smaller cables and connectors, which is kind of nice when you are managing the number of drives companies cram into server closets and rooms.

Another perk of SAS is that it is compatible with much cheaper SATA drives. This allows servers to be set up with SAS drives for frequently used data, supplemented with cheap, bulk, SATA storage. While the points about using thousands of drives may not be of interest to the home and office PC or even workstation user; the idea of mix and matching SCSI like speed at a lower price, with smaller cables, alongside comparatively dirt cheap SATA for their bulk storage, probably has a few peoples wheel’s turning.

While SAS improves SCSI dramatically, it is still clearly a server oriented technology. The first consideration is that SATA 3Gb/s is plenty fast for consumer rotational hard drives. The only reason to consider SAS is if you are laying the ground work for a server grade hard drive. These aren’t cheap (about 5 bucks US a Gig, as opposed to 11 cents a Gig for a normal 7.2k, 1TB drive). But they are the fastest rotating hard drives out there, comparable or even better than a solid state drive in some performance metrics. Don’t forget that unless you own one of the very few, rather expensive motherboards that include a SAS controller (like the 360 USD ASUS P6T6) you will need to get one to put in a PCI-e slot. Plan on spending a couple hundred bucks there.

That is a lot of money to spend, but the performance is there. The problem is, the performance is skewed for server tasks. Server disks need to zip around, getting lots of small pieces of data for lots of users. PC disks often save and load larger files, making the ability to track down data less important relative to how fast it can move it. Other high-performance options aimed at PCs offer arguably better performance for single users in absolute terms, and obviously far better performance on a per-dollar basis.

Western Digital’s Velociraptor Series brings 10k rotational speed to the desktop using a SATA connection, and leverages it to get the best performance not for a server, but a single user PC. $230 at Newegg for the 300GB version brings the per-gig price to .77 USD, without needing a fancy controller. While much more expensive than a normal drive, on a good day the Velociraptors can give SAS drives a run for their (significantly more) money.

If that still isn’t fast enough, you can go SSD. Intel’s X25-M is 80 GB and will run you $500, for a per gig price of 6.25 USD. That could be more than SAS, depending on what your SAS controller costs you. Falling SSD prices and increasing performance of solid state drives will be something to watch for through 2009.

It is a valid question: if drives, particularly SSDs, keep getting faster, should I track down one of those rare SAS having motherboards just in case? Good question, but no. By the time the speed of drives has outgrown the current SATA 3.0Gb/s standard, the SATA 6.0Gb/s standard, with products arriving as early as this year, will be established. It will offer desk and laptop users all the speed they need, even the enthusiast types. SAS is also launching a 6.0Gb/s standard this year, but like the first version of SAS, not just its cost, but its configuration and the types of advantages it provides (like having thousands of drives) will leave it decidedly a server solution.