Now that we have highlighted several of the attractive performance benefits of NVMe, we will dig into how NVMe provides those results.
One of the best aspects of PCIe is its wide deployment into the computing space. Replacing or supplementing other protocols requires the broadest use case possible. PCIe is pervasive in almost all aspects of computing, from laptops and mobile applications all the way up to data-center class systems. PCIe has a clear path forward with faster versions, such as PCIe 4.0, already in the making. This will unlock faster speed with each new generation.
The ubiquitous PCIe connection also provides a big jump in bandwidth. SSDs outstripped the 6Gb/s SATA connection almost on arrival, leading to a necessary rethinking of the spec. SAS 12Gb/s is conspicuously absent from this chart, and tops out around 1.2 GB/s.
One of the most impressive benefits to the PCIe connection is its ability to process read and write operations simultaneously (full duplex). The commonly used SATA interface conducts each type of operation separately (half duplex), creating a bottleneck that hinders saturation of SSDs parallelism characteristics.
PCIe also provides linear performance scaling. There is 1Gb/s of bandwidth available for each lane of PCIe Gen 3 (x1), and increasing the number of lanes increases speed until the lanes are all occupied. This creates a performance ceiling of 8000 MB/s for current generation devices.
The innate capabilities of NVMe are well suited to datacenter applications. NVMe's initial market penetration will focus on enterprise applications, much like the progression from PATA to SATA.
The stage is being set for client applications with the development of standardized connections and form factors, such as M.2, SATA Express, and SFF-8639 already well under way. The numerous connection methods will support backwards compatibility with AHCI, but the true advantages will come from use of the NVMe protocol. The HHHLx4 PCIe CEM 2.0 specification provides plenty of room to scale capacity, especially as we enter the 3D NAND generation.
The SFF-8639 is the Swiss army knife of connectors. It will support SAS, SATA, PCIe x4 SSDs, and backplanes for x4 PCIe SSDs and SAS/SATA HDDs. The SFF-8639 connector will bring the PCIe connection to the familiar 2.5" form factor for easily serviced and hot-swappable SSDs.
There are no plans to expand the SATA bus to a faster speed. Instead, there is a focus on SATA Express (SATA 3.2). SATA Express actually isn't a protocol, it is a dual SATA/PCIe connector. It supports SATA as well as a PCIe connection and can use either AHCI or NVMe as the logical device interface. SATA Express will transfer over to the SFF-8639 receptacle in the future.
NVMe's primary competitor will also leverage the SFF-8639 connector, and the x4 SAS noted in the graphic denotes the SCSI Express protocol. SCSI Express utilizes the SCSI command set over the PCIe bus by utilizing two T10 standards, SCSI over PCIe (SOP) and the PCIe Queuing Interface (PQI). The broad support for NVMe, and products already reaching the market, does not bode well for SCSI Express. SCSI Express is still being pushed to completion by the STA (SCSI Trade Association).
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