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Pushing For 6G SAS

Arthur Cole

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Question: LSI, Dell and Seagate recently demonstrated a 6G SAS network for the SCSI Trade Association. Was this a significant milestone?

So: This was the first interoperable demonstration of a 6Gbps SAS network featuring every component of the I/O path - server, HBA, SAS expander and disk drive. It was a validation of the development efforts of all three companies trying to bring the most robust storage solution to the enterprise industry. It paves the way for 6G SAS in late 2009.

Question: What makes 6G SAS preferable to 8G Fibre Channel or 10 GbE?

So: SAS was not meant to replace either Fibre Channel or iSCSI. It was initially developed as the next-generation SCSI, replacing parallel SCSI in DAS solutions. But the main disadvantage of both GbE and Fibre Channel is cost. SAS is a cheaper interface to implement, and faster too. A standard external connection typically holds four SAS physical layers. Each SAS PHY today is 3Gbps, which gives you 24Gbps. Upping that to 6G delivers a 24Gbps connection.

We are seeing interest in SAS on a host interface as a replacement for Fibre Channel in lower-price bands. SAS offers better performance at lower costs than other technologies, but it's limited by cable distances. 3G SAS tops out at about eight meters. With 6G, we're looking at 10-meter cabling. But the significance here is that we're doubling the transfer rate and still getting longer distances. With parallel SCSI, the line lengths got shorter as the bandwidth went up.

Question: So we could be seeing an entirely new SAN solution then?

So: We're seeing interest in 6G SAS as a switched topology, in blade centers especially. We're looking at a switch product for 3G, and we're moving forward on a product for 6G. There's also new interest in SAS as a host interface for external storage, primarily as a SAS-to-SAS interface for external RAID boxes. We'll still have FC-to-FC and iSCSI-to-SAS, but the interest is there for SAS-to-SAS.

The fact is, 6G SAS really doesn't make sense until the entire infrastructure is G. What's the point of having 6G if it's going to bottleneck as soon as it hits the 3G infrastructure? With 6G, you get greater reliability in the link - a more reliable link at a faster rate. You also get new features like decision feedback, equalization and standardized zoning that eliminates the compatibility issues SAS had when each vendor implemented zoning in their own unique way.

Read Arthur's recent blog entry “Big Plans for 6Gbps SAS" at


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