SAS-4 Features and the Storage Ecosystem

The next generation of Serial Attached SCSI – 24G SAS or SAS-4 technology is on track for a commercial launch in 2018. This revision of the standard includes protocol enhancements and new features that are targeted to take advantage of the improvements in media technology and drive higher performance across the storage sub-system.

SAS-4 doubles the link bandwidth by increasing the speed of operation to 22.5Gbps and changing the underlying encoding scheme from 8b/10b (dword mode) to 128b/150b (packet mode). It uses 20 bits of Forward Error Correction (FEC) and is designed to support a channel loss of 30dB. The physical layer specification leverages the 25G Common Electrical I/O specification published by the Optical Internetworking Forum which simplifies channel modelling and analysis. The protocol improves the Bit Error Rate (BER) by facilitating active adjustment of the Transmitter PHY to compensate for variations in channel loss due to any changes in the operating conditions. SAS-4 is backward compatible with two generations of the standard – 12Gbps SAS and 6Gbps SAS which means legacy equipment can co-exist and benefit from the bandwidth aggregation and performance enhancements of SAS-4 technology. It’s well proven and robust feature set enables unparalleled scalability, reliability and manageability and continues to make it the connectivity technology of choice for enterprise as well as cold storage architectures.

As CPU Host connectivity moves from PCIe Gen 3 to Gen 4, SAS-4 delivers bandwidth matching between the storage and the compute interface. It maximizes performance through bandwidth aggregation for legacy end points (e.g. high capacity HDDs) and bandwidth matching for high performance end points (e.g. high bandwidth SSDs). Breakthrough innovation in hard drive technology has made it possible to address the need for improved performance and greater storage capacity at affordable prices. SAS-4 introduces new features that can successfully harness the value proposition that these emerging drive technologies provide. Protocol improvements like enhanced fairness, intelligent connection management and buffering and low power modes enable the design of high performance storage sub-systems that provide substantial value to the end user. SAS-4 deployment is being supported by a strong ecosystem of companies that have brought to market the connector and cabling technology required to support 24G operation. In addition to new connector definitions, the current generation of 12G internal and external connectors have been upgraded to support the higher signaling rate. Their mechanical footprint remains the same which enables backward compatibility with legacy equipment. 24G test equipment that enables comprehensive verification and analysis of SAS-4 designs is currently production released.

Microchip is excited to offer controller and expander devices that are SAS-4 capable. If you would like additional information, please visit SAS-4 Features and the Storage Ecosystem to sign up for the whitepaper.

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