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M.2 SSD: Small, Embedded Flash Memory

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What is M.2?

M.2 solid state drives (SSD) are internal circuit boards that embed flash memory chips and controllers on thin circuit boards with a range of lengths and widths. M.2’s small and light form factor enables flash manufacturers to minimize flash storage sizes in laptops and workstations, and more recently in high-performance servers.

Under its original name Next Generation Form Factor (NGFF), M.2 set out to replace mSATA drives, which are thin circuit boards that fit onto a logic board or motherboard. The mSATA standard uses PCIe Mini Card design and connectors to enable SSDs on thin laptops. Some devices still use mSATA, and M.2 has a SATA connector. However, the SATA designs have a top throughput speed of 6.0Gb/s, and M.2 is capable of much faster speeds with faster buses like PCIe x4. 

As noted above, the M.2 is a solid state drive; a key fact about this storage technology is that it lacks an enclosure. It’s a small form factor circuit board that is internally mounted in a laptop, desktop, or server. M.2 can connect to multiple types of buses, which are the data pathways between the M.2 and connected computing components. The most common M.2 buses include SATA 3.0 and PCI Express (PCIe); and USB 3.0, which is backwardly compatible with USB 2.0.

The M.2 spec allows for different physical sizes and storage capacities. Speed depends on the type of connected bus: M.2 SATA can support speeds up to 6.0Gb/s, and up to 1GB/s per lane with PCIe 4x and NVMe. The spec also covers two M.2 controllers: AHCI for SATA compatibility, or NVME for high SSD performance.

Also see: Best Solid State Drives.

Types of M.2 Connections

In server environments, the most popular connection types are SATA and PCIe.

M.2 SATA SSD

  • Improves on mSATA standard. M.2 was developed in response to the mSATA standard. M.2 standardized SATA into its new drive interface under SATA 3.2 specifications. The M.2 spec reduced length and width measurements while doubling storage capacity using more flash memory chips.
  • Still supports mSATA. Although M.2 SATA is distinctly slower than M.2 PCIe, SATA is widely deployed in enterprise applications and industrial settings. M.2 enables compatibility with SATA in a hot-swappable, very small form factor.

M.2 PCIe

The PCIe spec has been around for a while: standards group PCI-SIG introduced PCIe 1.0a in 2003. Today its most popular version is 4.0, and PCI-SIG has ratified PCIe 5.0.

  • PCIe interface standard. The PCIe standard connects high-performance computing components, including M.2 SSDs. Serial connections contain from 1 path to multiple data transmission paths, called lanes. Each lane independently connects the controller to another computing component, so a 4x lane transports data 4 times faster than a single lane.
  • PCIe and M.2. M.2 PCIe SSDs use PCIe data transfer lanes to accelerate data transfers. The NVMe controller connected to PCIe 4.0 4x enables the M.2 SSD to achieve very high performance. The configuration scales performance up to intensive database workloads with high transfer speeds and potentially thousands of processing queues. There is also a distinct performance boost depending on the number of PCIe lanes. An M.2 SSD with an NVMe controller on a PCIe x2 transfer data at about 15.75Gb/s. But double 2x to 4x, and the speed doubles to about 31.5Gb/s.
  • Clearing up the confusion. Sometimes an M.2 running PCIe and NVMe may be called an “M.2 PCIe SSD” or an “M.2 NVMe SSD” as if they were different products. They’re usually not, but this takes some explaining. NVME is a data transfer protocol that uses a PCIe bus. The M.2 spec allows manufacturers to put an NVME controller and PCIe 4x connector on an M.2 card. However, the NVMe/PCIe combination is not limited to M.2 form factors, and can run on other types of cards. M.2 NVMe or M.2 PCIe terms usually refer to the same type of SSD: an M.2 form factor running NVMe on a PCIe bus.

M.2 Connections and their Usage:

M.2 SSD

M.2 Size and Capacity

  • M.2 sizes. M.2 SSDs are rectangular. They are 22 millimeters (mm) wide and usually 60 mm or 80 mm long, although there are also 30 mm, 42 mm and 110 mm length cards. Longer length M.2 drives usually hold more flash memory chips for extra capacity than the shorter versions. The card size is identified by a four- or five-digit number. The first two digits are the width and the remaining numbers are the length. For example, a 2260 card is 22 mm wide and 60 mm long. The most common dimensions are 2242, 2260, and 2280.
  • Single or double? M.2 drives can also be single or double-sided. Most laptops use the 22 mm width and single-sided boards, whose thin profile fits into ultra-thin laptops. In environments where longer lengths would be an issue, the M.2 spec allows both single-sided and dual-sided versions to fit more components into a smaller length. The maximum thickness if 1.5 mm per side.
  • Size = storage capacity. Servers may also use the 22 mm width, but are better fits for longer lengths and/or double-sided boards. These factors hold more chips, which increases M.2 storage capacity. For example, 80 to 110 mm lengths can house up to 8 NAND chips with a typical 1TB in storage capacity, although some manufacturers like Samsung have reached 2TB in capacity on the M.2 form factor.

M.2 Speed and Performance

M.2 transfer speeds

The M.2 specification is used on SSDs with some fast read write times.

  • It’s not the size. M.2 sizes have everything to do with capacity thanks to more flash memory chips, but have little to do with performance speed. M.2 SSD performance depends on the speed of the bus and the number of PCIe paths.
  • A world of difference. A SATA M.2 is limited to SATA speeds, with typical SATA SSD read/write speeds coming in around 500-550MB/s. If the M.2 is attached to NVMe and PCIe, write speeds reach as high as 3500MB/s.

Keys and Sockets

  • Module keys. M.2 form factors not only have different dimensions and sizes; they also come with different physical connectors. Each type of connector has a different module key, and each key exposes different interfaces. Keys can connect directly to USB 3.0 and SATA, but may need an adapter card for PCIe/NVMe. For example, the Samsung 970 EVO is an M.2 PCIe SSD that can connect directly to a PCIe slot, or it can use an adapter card to fit into a PCIe x4 slot.
  • Keying connections. M.2 manufacturers key the SSD so users cannot insert the wrong M.2 card connector to the host’s card sockets. Key types are labeled on the edge connectors of the SSD, and help users to make proper connections. For example, M.2 SATA using AHCI can fit in B- and M-keyed modules. M.2 NVMe for PCI 2.0 x4 are only M-keyed. Different key configurations exist with different interfaces.
  • Sockets. The sockets have their own keys, and are not interchangeable. Socket 1 serve wireless connections, Socket 2 serve different connections such as GPS solutions and cache configurations, and Socket 3 is the drive socket for M.2 SATA or PCIe SSDs.

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