SAS 12Gb/s Architecture
The emergence of mega-datacenters and high-density deployments has required more connectivity features from SAS, while the simultaneous explosion of flash into the datacenter has required massive performance improvements at a pace unseen in the recent history of storage technology.
The regular update cadence of the SAS interface has provided a massive performance jump of 6-10X in the last three years. This rapid upgrade schedule and wide deployments will help SAS survive the challenge from new emerging specifications. While these new specifications have a number of appealing enhancements, they are still in their infancy with no working silicon in general availability.
One of the keys to success for the SAS interface has been existing infrastructure and backwards compatibility. The lack of backward compatibility, and infrastructure changes that could be required in some cases, can hinder widespread adoption of any new interface. Many refer to SAS as the 'Ethernet of storage'. Like Ethernet, SAS has been challenged by many different protocols over the years, but fast upgrade cycles and support for existing infrastructure have helped it weather those storms.
12Gb/s SAS is now entering general availability with HBA's and RAID controllers, and the new storage products to connect to them, being released over the next several months. The evolution to 12Gb/s SAS will continue for several years. The first 24Gb/s Plugfest is slated to begin in 2016, followed by end-user availability in 2017. This gives 12Gb/s SAS roughly four years before it is superseded by a more robust SAS protocol.
Aside from the bandwidth expansion, which allows 12Gb/s to fully saturate the PCIe 3.0 bus at 8,000MB/s, the upgraded architecture also brings new connectors that provide more expansive connection capabilities and enhanced density.
The new Mini-SAS HD (High Density) SFF-8643/8644 connections provide both an active and passive scheme for 6Gb/s and 12Gb/s SAS. These new connectors also sport an electrically improved design with less cross-talk and a better signal to noise ratio.
Adaptec recently switched to the HD connectors with their line of 6Gb/s products and other RAID and HBA vendors are expected to make the switch to Mini-SAS HD with 12Gb/s products. The smaller connector allows the connection of more devices to the smaller HHHL (Half Height, Half Length) controllers that are becoming commonplace in today's dense server deployments.
The expanded speed of 12Gb/s SAS will also raise the current cap of 24GBps throughput of a normal 4 lane SAS cable up 48GBps. This will enhance density by allowing the connection of more devices per expander before the connection between the HBA/RAID controllers becomes the bottleneck.
The SAS Connectivity Roadmap also points to a massive upgrade in cabling distances, with 20m for active copper and up to 100 meters with optical connections. This will provide a means for SAS to support box-to-box, server-to-storage, and rack-to-rack connections. In conjunction with SAS switches and expanders, active copper and optical can create an entirely new ecosystem within the datacenter. These new capabilities will enable expanded topologies for both custom builders and modular applications.
Improving the connectivity and ease of operation is a tall order, but the new Mini-SAS HD connectors deliver a spate of enhanced functionality. SAS Connectivity Management supports connection discovery and cable management by detecting the presence (or absence) of passive, active and optical connections. Along with rapid fault isolation and the minimization of configuration errors, this can simplify large-scale deployments significantly.
The only initial inhibitor for Mini-SAS HD will be the significantly higher price of cabling for the next few months. As we hit general availability and the economies of scale factor in, there should be significant easing of the pricing structure. Overall, the widespread adoption of these new connections and features with the 12Gb/s generation of storage devices will simplify and expand the capabilities of SAS; allowing for greater distance, scalability, usability and serviceability for large-scale deployments.
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