It is plain to anyone who takes interest in SSDs that raw bandwidth has become bottlenecked by the SATA 3 interface. Capable of 6Gbps (550MB/s) optimally, SSDs are routinely hitting this figure both on read and write rates, to the extent that competition is now largely through pricing and higher IOPS. Something had to give, and the SATA steering committee eventually finalised the next generation of SATA – SATA Express.
SATA Express was conceived as a backwards compatible standard that added support for PCI-Express signalling, removing a layer of abstraction between the system bus and storage controller. PCI-Express was already proven as a viable system, and bringing it into the consumer space makes a lot of sense from a speed and cost perspective. By tying in two PCIe 2.0 lanes the protocol is capable of up to 1GB/s (2 x 5GT/s with 8b/10b encoding) in both directions when connected to a suitable device and can be scaled to PCIe 3.0, but timing tricks and robust cable shielding is necessary to ensure high speed and reliability. The physical connector is also bulky, a trade-off to enable backwards compatibility.
M.2, formerly known as the Next Generation Form Factor (NGFF), builds on the SATA Express specification by implementing a standard for internally mounted expansion cards including high speed storage. Broadly speaking it replaces and surpasses mSATA, with functions that encompass and build upon mSATA to include PCI-Express and USB 3.0 signalling. In the process it is more space efficient than its predecessor, including variable lengths, widths and allowing components on both sides of the PCB.
The current M.2 definition document outlines support for signalling via SATA, SATA-Express via ACHI, and SATA-Express via NVMe, the latter of which is a protocol designed specifically for solid state storage but is currently unsupported by a majority of SSD controllers and operating systems. For this reason some M.2 devices will top out at 6Gbps read/write rates due to legacy compatibility, others will be capable of up to 1GB/s thanks to a native PCI-Express storage controller.
The M.2 connector has 75 pin positions, some of which are removed as keying notches depending on the use case of the attached card.*
There is a huge potential for variation in the dimensions of M.2 devices but a width of 22mm and 30/42/60/80/110 mm lengths are the most common packages. Motherboards which support M.2 are typically compatible with 42-80mm, whilst others should be taken on a case-by-case basis.
Theoretical speeds for M.2 storage devices utilising PCI-E signalling are significantly higher than SATA3's 6Gbps (~550MB/s). Initial solutions utilise two PCI-E 2.0 lanes for a total bandwidth of 1GB/s, whilst draft specifications outline support for as many as four PCI-E 2.0 lanes or extend support to PCI-E 3.0, effectively doubling the maximum available bandwidth to 2GB/s. The flexibility of the standard also allows for additional modifications to be made at a later date should it prove necessary.
M.2 therefore should be a hit in the embedded, mini-PC and laptop arenas where high performance in a miniature form factor is a major selling point. It's role on Z97 motherboards is less clear except that it provides next-generation storage speeds for devices which are already on the market, whereas the alternative SATA-Express connector has no comparable drives available to consumers.
*M.2 Connector pin-out via http://uk.farnell.com/images/en_UK/pdf/TE_M.2datasheet.pdf
