The Need For Speed

Today’s SAS and SATA drives transfer data at rates of 6 Gbits/s, but as microprocessor and dynamic RAM subsystems get faster, still higher-speed interfaces will be needed to ensure the memory systems don’t get starved for data. SAS drives that can transfer data at double the rate – 12 Gbits/s – are already being sampled, and there is already some exploratory work looking at a 24 Gbit/s interface. However, just doubling the data rate is not enough – designers need faster interfaces and tiered storage solutions to get the best system-level throughput. Thus, designers have started to show the next-generation storage interface – SCSI Express.

Able to operate at data transfer rates of 13.1 Gbits/s SCSI Express runs the SCSI protocol over a single-lane PCI Express (PCIe) hardware interface (SOP – SCSI over PCIe). Multiple PCIe lanes can also aggregated to achieve still higher data transfer rates (See the diagram). SCSI Express is an industry initiative that leverages the popular PCIe hardware interface and the PCIe architecture queuing interface (PQI) model that the T10 technical committee in the SCSI Trade Association is defining.

At a recent meeting of the SCSI Trade Association in Santa Clara, association members demonstrated the upcoming next-generation storage interface that will leverage PCI Express version 3.0, which will permit data transfers at 13.1 Gbits/s, and drives that support the new 12-Gbit/s serial-attached SCSI interface. Drives from Seagate Technology and Western Digital were demonstrated, as well as an FPGA implementation of prototype storage controller chip from SanDisk, and a protocol analyzer from LeCroy that can do full SCSI Express decoding. And, to support the box-to-box connections, cable vendors demonstrated passive and active copper cables, as well as optical cables.

By combining the SAS-based drives with the robust SCSI protocol and PCIe 3.0 , the SCSI Trade Association has defined a backplane dubbed Express Bay that can accept SATA, SAS, MultiLink SAS and SCSI Express storage devices. The backplane employs a multi-protocol connector that can accept SAS or SCSI Express drives and provide power of up to 25 W for high-performance solid-state drives (SSDs), thus permitting system integrators to craft tiered storage systems that combine hard-disk drives and SSDs to accelerate system performance.

Additionally the combination of the high-speed PCIe interface and 12 Gbit/s serial-attached SCSI on a common backplane system will allow architects to combine the best performance aspects of both storage interfaces to deliver high-performance storage arrays that can feed data to the high-speed computational systems. The 12 Gbit/s SAS interface better utilizes the bandwidth of the PCIe 3.0 bus, and even the older 6 Gbit/s drives can leverage the higher bandwidth of PCIe 3.0 by using a feature known as “Store and Forward” in which the data streams from two slower drives are aggregated to form a 12 Gbit/s data stream.

Dave Bursky
Technology Editor,
Chip Design Magazine

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