M.2 (pronounced M dot two)
formerly known as the Next Generation Form Factor (NGFF), is a specification for internally mounted computer expansion cards and associated connectors. It replaces the mSATA standard, which uses the PCI Express Mini Card physical card layout and connectors. M.2’s more flexible physical specification allows different module widths and lengths, and, paired with the availability of more advanced interfacing features, makes the M.2 more suitable than mSATA for solid-state storage applications in general and particularly for the use in small devices such as ultrabooks or tablets.
Computer bus interfaces provided through the M.2 connector are PCI Express 3.0 (up to four lanes), Serial ATA 3.0, and USB 3.0 (a single logical port for each of the latter two). It is up to the manufacturer of the M.2 host or device to select which interfaces are to be supported, depending on the desired level of host support and device type. The M.2 connector has different keying notches that denote various purposes and capabilities of M.2 hosts and modules, preventing plugging of M.2 modules into feature-incompatible host connectors.
In addition to supporting legacy Advanced Host Controller Interface (AHCI) at the logical interface level, M.2 specification also supports NVM Express (NVMe) as the logical device interface for M.2 PCI Express SSDs. While the support for AHCI ensures software-level backward compatibility with legacy SATA devices and legacy operating systems, NVM Express is designed to fully utilize the capability of high-speed PCI Express storage devices to perform many I/O operations in parallel.
A high-level overview of the SATA Express software architecture, which also applies to M.2. It supports both legacy SATA and PCI Express storage devices, with AHCI and NVMe as the logical device interfaces.
Buses exposed through the M.2 connector are PCI Express 3.0, Serial ATA (SATA) 3.0 and USB 3.0, which is backward compatible with USB 2.0. As a result, M.2 modules can integrate multiple functions, including the following device classes: Wi-Fi, Bluetooth, satellite navigation, near field communication (NFC), digital radio, Wireless Gigabit Alliance (WiGig), wireless WAN (WWAN), and solid-state drives (SSDs). The SATA revision 3.2 specification, in its gold revision as of August 2013, standardizes the M.2 as a new format for storage devices and specifies its hardware layout.
The M.2 specification provides up to four PCI Express lanes and one logical SATA 3.0 (6 Gbit/s) port, and exposes them through the same connector so both PCI Express and SATA storage devices may exist in form of M.2 modules. Exposed PCI Express lanes provide a pure PCI Express connection between the host and storage device, with no additional layers of bus abstraction. PCI-SIG M.2 specification, in its revision 1.0 as of December 2013, provides detailed M.2 specifications.
There are three options available for the logical device interfaces and command sets used for interfacing with M.2 storage devices, which may be used depending on the type of M.2 storage device and available operating system support
Used for SATA SSDs, and interfaced through the AHCI driver and legacy SATA 3.0 (6 Gbit/s) port exposed through the M.2 connector.
PCI Express using AHCI
Used for PCI Express SSDs and interfaced through the AHCI driver and provided PCI Express lanes, providing backward compatibility with widespread SATA support in operating systems at the cost of not delivering optimal performance by using AHCI for accessing PCI Express SSDs. AHCI was developed back at the time when the purpose of a host bus adapter (HBA) in a system was to connect the CPU/memory subsystem with a much slower storage subsystem based on rotating magnetic media; as a result, AHCI has some inherent inefficiencies when applied to SSD devices, which behave much more like DRAM than like spinning media.
PCI Express using NVMe
Used for PCI Express SSDs and interfaced through the NVMe driver and provided PCI Express lanes, as a high-performance and scalable host controller interface designed and optimized especially for interfacing with PCI Express SSDs. NVMe has been designed from the ground up, capitalizing on the low latency and parallelism of PCI Express SSDs, and complementing the parallelism of contemporary CPUs, platforms and applications. At a high level, primary advantages of NVMe over AHCI relate to NVMe’s ability to exploit parallelism in host hardware and software, based on its design advantages that include data transfers with fewer stages, greater depth of command queues, and more efficient interrupt processing.
Form factors and keying
An M.2 socket on a computer motherboard, visible in the upper-left portion of the picture. The socket is keyed in the M position and provides two positions for the mounting screw, accepting 2260 and 2280 sizes of M.2 modules.
The M.2 standard has been designed as a revision and improvement to the mSATA standard, with the possibility of larger printed circuit boards (PCBs) as one of its primary incentives. While the mSATA took advantage of the existing PCI Express Mini Card (Mini PCIe) form factor and connector, M.2 has been designed from the ground up to maximize usage of the PCB space while minimizing the module footprint. As the result of the M.2 standard allowing longer modules and double-sided component population, M.2 SSD devices can provide larger storage capacities and can also double the storage capacity within the footprints of mSATA devices.
M.2 modules are rectangular, with an edge connector on one side (75 positions with up to 67 pins, 0.5 mm pitch, pins overlap on different sides of the PCB), and a semicircular mounting hole at the center of the opposite edge. Each pin on the connector is rated for up to 50 V and 0.5 A, while the connector itself is specified to endure up to 60 mating cycles. The M.2 standard allows module widths of 12, 16, 22 and 30 mm, and lengths of 16, 26, 30, 38, 42, 60, 80 and 110 mm. Initial line-up of the commercially available M.2 expansion cards is 22 mm wide, with varying lengths of 30, 42, 60, 80 and 110 mm.
An M.2 module is installed into a mating connector provided by the host’s circuit board, and a single mounting screw secures the module into place. Components may be mounted on either side of the module, with the actual module type limiting how thick the components can be; the maximum allowable thickness of components is 1.5 mm per side. Different host-side connectors are used for single- and double-sided M.2 modules, providing different amounts of space between the M.2 expansion card and the host’s PCB. Circuit boards on the hosts are usually designed to accept multiple lengths of M.2 modules, which means that the sockets capable of accepting longer M.2 modules usually also accept shorter ones by providing different positions for the mounting screw.