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SAS 2.0 improvements spur shift from Fibre Channel drives to SAS drives

SAS 2.0 increases interface speed to 6 Gbps, improves connectivity and facilitates better signal-to-noise ratio, which will propel the shift to SAS drives over next five years.

The second generation of serial-attached SCSI (SAS 2.0) increases the speed of the interface to 6 Gbps, improves connectivity to allow for greater storage density and facilitates a better signal-to-noise ratio to assist with the transition from Fibre Channel drives to SAS drives.

The changes will help to make SAS the dominant interface within data storage systems by the middle of the decade, according to John Rydning, a research director for hard disk drives (HDDs) at Framingham, Mass.-based IDC.

In this podcast interview, Rydning discusses the significance of the SAS-2 changes and offers recommendations on how IT shops can plan for the future while keeping their disk drive options in mind.

You can read the transcript that follows or listen to the MP3 file below:

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Rydning: There have been two important recent updates to the SAS interface standard that I think are meaningful and important for end users to understand. The first significant change is the increase in the speed of the interface from 3 Gbps to 6 Gbps. That change was announced in the fourth quarter of 2008, and some of those products are shipping today.

The second update to the SAS-2 interface standard, often referred to as SAS 2.1, improves SAS device connectivity. There have been two important updates on SAS 2.1. The first is a change of the interconnect. There's now a small form-factor high-density (HD) connector, often referred to as the mini-SAS HD connector, that allows for greater storage device density in enclosures, especially those using 2.5-inch enterprise-class disk drives< or similar form-factor solid-state drives (SSDs). The SAS 2.1 standard also reduces electrical signal crosstalk to provide a better signal-to-noise ratio over connections. This is important for storage systems that might make that transition from a Fibre Channel to a SAS architecture.

So, if there are some end users that are currently using Fibre Channel today, I think they'll find that the SAS 2.1 interface standard will help make that type of technology more usable for them in the future. What adoption trends have you seen with SAS vs. other types of disk drives, and what do you expect the breakdown to be five years from now?

Rydning: In enterprise applications, we see three different interfaces being used on the storage devices. Today, there's either SAS or Fibre Channel and even SATA. Now the SATA interface is used mainly on what we call capacity-optimized storage devices or capacity-optimized hard drives. These drives are used to provide high-capacity yet reliable 24/7 storage in rack-mount applications. But they are not designed for what we call high input/output performance in transactional environments.

If users are looking for high I/O performance, they can look to performance-optimized disk drives or solid-state drives as solutions. About 20% of the performance-optimized HDDs shipping today still ship with a Fibre Channel interface, and about 80% now are shipping with a SAS interface. As I mentioned, that's making a transition from the 3 Gbps to the 6 Gbps transfer rate.

We expect that by the second half of this year, the majority of the SAS HDDs shipping will be SAS 6 Gbps products. And, by 2013, Fibre Channel will be only shipped for after-market sales and upgrading existing systems. So, by the middle of this coming decade, SAS is going to be the predominant interface inside storage systems. What are the pros and cons of the SAS architecture vs. Fibre Channel?

Rydning: First, let me say that there is nothing wrong with the reliability and performance of Fibre Channel system architectures and Fibre Channel storage devices or the interface itself. Fibre Channel has been a stalwart technology for more than a decade now, especially in SAN environments.

But going forward, 6 Gbps SAS components, including the drive controllers and the host bus adapters, are all expected to grow into very high-volume products, and that will provide economies of scale that just will not be possible with Fibre Channel components.

I think cost is going to be the most important pro associated with SAS 6 Gbps technology components. The only potential con I can see with SAS 6 Gbps products is managing the signal integrity on backplanes and over long distances. And, as I mentioned already, there are some new standards that are addressing those concerns. What advice would you like to leave with IT shops about their disk drive options as they plan for the future?

Rydning: I think IT shops need to realize that there are no further data rate performance improvements planned for Fibre Channel storage devices; 4 Gbps Fibre Channel is as fast as it gets. The HDD industry is not planning to transition to either 6 [Gbps] or 8 [Gbps] or any higher data transfer rates for Fibre Channel products.

In contrast, the SAS technology roadmap is showing 6 Gbps speeds today and 12 Gbps in the future. Further, as I mentioned already, Fibre Channel is going to become a legacy technology, especially, again, inside enclosures over the next five years. It's going to be supported, but it's only going to be supported in legacy applications.

Given these trends, I think it would be prudent for IT shops to begin making the plans to transition to SAS architectures, especially as they consider making replacements or upgrades to their systems over the next two or three years.


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