Definition

U.2 SSD (formerly SFF-8639)

Contributor(s): Carol Sliwa

A U.2 SSD is a high-performance data storage device designed to support the Peripheral Component Interconnect Express (PCIe) interface using a small form factor (SFF) connector that is also compatible with standard SAS and SATA-based spinning disks and solid-state drives (SSDs). Although U.2 SSD technically could describe a PCIe, SAS or SATA drive, manufacturers generally use the term only to describe a 2.5-inch PCIe SSD that supports the nonvolatile memory express (NVMe) host controller interface and storage protocol. NVMe technology can speed the transfer of data between host systems and SSDs over a PCIe interconnect.

The main impetus for the development of U.2 was to enable the newer and faster NVMe-based PCIe SSDs to plug in to the same drive backplane as legacy SAS and SATA drives without having to shut down an enterprise server and storage system to insert or remove them.

History of the U.2 form factor

The SSD Form Factor Working Group, comprised of vendors with commercial interests in SSD technology, formed in 2010 to promote PCIe as a high-speed interconnect. Group members developed a connector specification to enable PCIe to interoperate with the SAS and SATA storage protocols in a disk enclosure designed for standard 2.5- or 3.5-inch drives. The specification defines the physical connector's mechanical and electronic properties, power profiles and hot-plug design.

Founding members of the SSD Form Factor Working Group that developed the U.2 connector specification included Dell and EMC -- before the companies merged -- Fujitsu, IBM and Intel. Other members contributing to the connector specification included Amphenol, Emulex (since acquired by Broadcom), Fusion-io (acquired by SanDisk and later Western Digital), Integrated Device Technology, Marvell, Micron, Molex, PLX Technology (acquired by Broadcom), QLogic (acquired by Cavium), SandForce (acquired by LSI, then Avago Technologies and later Seagate Technology), Smart Modular Technologies and sTec (acquired by HGST and then Western Digital).

The SSD Form Factor Working Group chose U.2 as the marketing term to describe PCIe, SAS and SATA profiles for the SFF-8639 connector. The SFF-8639 connector can support one SATA port, two SAS ports or up to four lanes of parallel I/O in PCIe SSDs. The drive form factor can be 2.5-inch or 3.5-inch.

SATA, SAS, U.2 backplanes

A subsequent SFF-9639 specification defines the pinout information for the various interfaces designed for use with the SFF-8639 connector. Available profiles include U.3 and USB in addition to U.2's PCIe, SAS and SATA. The Storage Networking Industry Association (SNIA) SFF Technology Affiliate Technical Work Group now has purview over the SFF-9639 specification after merging with the group that did the original work on the SFF-8639 and SFF-9639 connectors.

Advantages/disadvantages

One of the main advantages of U.2 SSDs over typical add-in cards (AICs) that plug in to a computer's internal PCIe bus is that users do not need to open a server to insert or remove them. They are hot-pluggable whether or not the system receives prior notification, just like SATA and SAS SSDs and hard disk drives (HDDs).

Another key advantage of the U.2 connector and form factor is backward compatibility with HDDs. Server and storage manufacturers are able to configure enterprise systems with a mix of 2.5-inch or 3.5-inch HDDs and PCIe-, SAS- and SATA-based U.2 SSDs to suit the needs of their customers. The system's backplane must be designed to support NVMe in order to use the NVMe-based PCIe SSDs.

A U.2 SSD supports up to four lanes of parallel I/O over a PCIe connection in an enterprise server or storage system. With a dual-controller storage array, a pair of two-lane PCIe links would operate independently of each other, allowing for the failure of one controller. A defined signal on the SFF-8639 connector indicates to the drive whether to operate in single- or dual-port mode.

The U.2 SSD specification offers optional support of the System Management Bus (SMBus) protocol to monitor the voltage and temperature of the motherboard's control or system management controller.

One potential disadvantage of U.2 SSDs is the higher cost of the drive technology, including the SFF-8639 connector, so users need to determine if they need the higher performance and lower latency that NVME-based PCIe SSDs can enable. Enterprise users also may need to buy new servers designed to support U.2 SSDs, since their legacy infrastructure may not be equipped with the SFF-8639 connector technology.

U.2 SSD vs. M.2

Two types of SSDs that have gained popularity in conjunction with the escalating interest in high-performance, low-latency NVMe technology are M.2 and U.2.

SFF M.2 SSDs were originally designed for thin, power-constrained devices, such as tablet computers and laptops. Single-port M.2 SSDs also see use as boot devices and storage drives in web and other servers that do not require dual-ported drives for enterprise use. The M.2 specification supports PCIe, SATA and USB interfaces.

Manufacturers produce M.2 SSDs in a variety of sizes. M.2 SSDs are typically 22 mm in width and 60 mm, 80 mm or 110 mm in length.

SSD form factors
This image shows 2.5-inch SAS and PCIe U.2 SSDs, SATA SSDs, PCIe AICs and M.2 SSDs.

By contrast, U.2 SSDs are designed to plug in to the 2.5- and 3.5-inch slots in the backplanes of enterprise server and storage systems. U.2 SSDs also can enable a higher maximum storage capacity than M.2 SSDs as a result of their larger physical size.

The U.2 form factor is more conducive to heat dissipation than M.2 SSDs and permits higher operating temperatures without damaging the drive components. That feature helps to keep a U.2 SSD from throttling performance when the drive heats up during write-intensive operations.

U.2 and some types of M.2 SSDs can be swapped in or out of a running computer system. But the system layout must enable open access to the M.2 SSDs to make the hot-pluggability feature useful from a practical standpoint.

This was last updated in April 2018

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