From SCSI to SAS: A Revolution in Enterprise Storage

Original Article Date: 2007-04-26

In this issue I wanted to finally write on a development that I'd keeping my eye on these last 12 months. It concerns SCSI, the primary technology behind enterprise storage, and its replacement SAS.

Like all major shifts in computing technology, judging when is the right time to jump is always tricky. Jump too soon, and it can bite you in the form of bugs not yet straightened out, high-prices, a lack of product choice, and general "v1.0" issues. Jump too late, and you can end up with hardware that is bordering on obsolete the moment you buy it.

It's my opinion that the time to jump to the new technology is right now, and from this point on, all Electronics Nexus' default choices for enterprise storage will utilise SAS instead of SCSI. But in order for you to make up your own mind, let's find out why SAS is such an advance over SCSI.

A Fond Farewell to SCSI

SCSI ("Small Computer Systems Interface") has been with us in a standardized form since the mid 1980s, so it has a long and proud history of providing business with reliable and fast enterprise storage. Despite the high costs of SCSI disk drives and controllers, network administrators favored them due to their exceptional reliability in comparison to IDE/ATA. Power workstation users enjoyed SCSI's outstanding performance due to comparatively high spindle speeds.

SCSI has evolved and improved through many versions, from the 5MB/s SCSI-1, all the way up to 320MB/s of Ultra-320 SCSI. But the basis of the standard has remained essentially unchanged, namely, that SCSI is an parallel interconnect bus that enables multiple drives to connect on to it.

One disadvantage of this evolution, however, is that with Ultra-320 SCSI today, we are still using a 1980s protocol. SCSI is notoriously difficult to setup, and is normally best left to the professionals. Each SCSI channel required each device connecting to it to have a unique ID (which had to be set manually using jumpers on the drive), and a terminator had to be connected to the each end of the cable to prevent reflection of stray signals. Heaven have mercy if you didn't do one of these two things correctly (very odd errors would occur which would involve lots of head-scratching and sometimes take days to even discover).

Further, the current limit of drives on a single SCSI channel is 15. But because all these drives were sharing a common bus, the 320MB/s limit of the specification could be reached after only 5 or 6 drives running flat out (especially if they were the higher-speed 15kprm models), an issue known as contention. To add more than 15 drives to a RAID required the spanning of two SCSI channels (which incidentally also provided the advantage of increasing the I/O bandwidth to 640MB/s), so even then your practical limit was confined to 30 drives. With media-serving and new legal-compliance for record keeping requirements dramatically increasing the need for massive online or nearline storage, the need for more than 30 drives in easily accessible and configurable disk arrays is there.

Add to that the issue of manufacturers needing to make two versions of the same drive - one with 68-pins for stand-alone usage, and one with 80-pins for use in hot-swap assemblies - and you can see we were dealing with a system that had flaws.

SAS is Serial Attached SCSI

A replacement was needed, and so a number of years ago, the SCSI Trade Association (a consortium of disk drive and controller manufacturers) came together to hash out a new standard. This new standard was called SAS, an abbreviation of Serial Attached SCSI.

The new specification set out to retain all the qualities of SCSI, but to handle the concerns outlined above. And it did so in the following ways:

• Serial point-to-point interconnect. No more shared bus, no more termination, SCSI ID or contention issues. Simple to connect, just plug in one drive to one port connector on the controller card using a slimline serial cable, like Serial ATA.
• Massive drive arrays possible. With the new "SAS Expander" Technology, each expander (usually located on a SAS hotswap backplane) can provide unique addresses and connectivity for up to 128 devices, whilst expanders can daisy-chain together to form mega-arrays of up to 16,384 drives, all off a single master controller!
• A single form factor for standalone or hotswap. Able to be connected as a stand-alone drive, or inserted into a hotswap assembly, a 3.5" SAS drive is always the same.

• Compatibility with SATA. SAS is a superset of SATA, so you can connect SATA-II drives into any SAS controller, and plug SATA-II drives into SAS backplanes. This enables system administrators to combine any combination of high-performance and system-critical SAS drives, with their higher capacity and better value cousins, for maximum flexibility and value.
• 3.5" and 2.5" form factors. Choose between standard 3.5" 15kprm drives for better value and higher performance, or select notebook sized 2.5" 10krpm drives for minimum volume profile in high-density applications such as blade servers. It is expected that with miniaturization in drive technology continuing that 2.5" drives will become the standard in the long-term.
• SAS is fully scalable. Because SAS is built upon a serial technology, it is much more scalable in terms of bandwidth capacity. Currently, 3Gb/s is the bandwidth limitation for individual SAS connections, but this is expected to increase to 6Gb/s by the end of 2008, and 12Gb/s by 2012.

Ready to Jump?

So it can be seen that SAS comes with many new benefits as well as fixing the issues of the old SCSI standard.

Up until a couple of months ago, however, SAS drives and controllers carried a premium of 15-20% over SCSI, and so the reasons for most users considering to move over to SAS were not compelling enough. But with SAS prices now at parity with SCSI, and with the drive and controller manufacturers now telling us SCSI will be effectively end of life by 2008, the spectre of building obsolete systems currently with SCSI looms large. With these final two factors in mind therefore, I feel it is time to now make the switch.

For many this will be a tough sell. SCSI has been with them for 20 years or more. And that reputation for reliability and performance is difficult to give up. But, although you could think of SAS as being a revolution in enterprise storage, it is really more of a step-change in evolution. SCSI is still the trusted model amongst discerning system administrators worldwide, and to throw-out all its qualities for the sake of change would have been wrong.

SAS is still SCSI, it's just been modernized for the 21st century, with a much easier to use and more versatile serial interface. The drives inside are still essentially the same. It's just how we're connecting them to the outside world that's different!

Ben Ranson
Chief Systems Engineer