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Connecting PCIe and storage devices

04 Feb 2014  | Josh Beaudet

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Solid-state drives (SSDs) are rapidly becoming the most preferred storage method. With this changing of the guard from hard-disc drives to SSDs, there is a need for different connections than are used for hard drives or peripherals to utilise the SSD's full potential. The connector and protocols discussed here are the future for the storage industry.

Peripheral Component Interconnect express (PCIe) has become the dominant connection technology between computer motherboards and internal peripheral devices such as graphics cards. The current generation (PCIe Gen3) operates at 8GT/s/lane on up to 32 lanes. PCIe has broken into the storage world with the advent of SATA Express and, NVMe (Non-Volatile Memory Express), storage technologies that use the PCIe connectors and signalling.

As computers have become smaller—such as Ultrathin notebooks or tablet PCs—connectors for peripheral devices had to shrink to accommodate tighter spaces. The PCI-SIG developed the M.2 connector to meet this demand. The connector is 22mm wide and the length can vary to make the best use of the technology being employed over it. The size is a 43% reduction in size over the standard x4 lane PCIe connector. The M.2 (figure 1) supports four lanes of PCIe traffic or one SATA port if in legacy mode. SATA Express is one of the newest technologies to take advantage of this connector.


Figure 1: The M.2 card was formerly known as the NGFF (New Generation Form factor) card. Source: sata-io.org


The SATA Express specification provides a means of using PCIe as a storage interface while maintaining the existing software infrastructure. There are two registers and command sets that SATA Express can choose from: AHCI (Advanced Host Controller Interface) and NVMe. AHCI is supported by all major operating systems, but it's not streamlined for SSDs (solid state devices). The NVMe register and command set is, however, streamlined for SSD use. SATA Express can take advantage of the PCIe M.2 connector inside computers. For SATA Express in enterprise storage applications, the SFF-8639 or the SATA Express connector (figure 2) can be used.


Figure 2: SATA Express connectors were designed for enterprise storage connections.



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