U.2 (pronounced "U-dot-2"), formerly known as SFF-8639, is a computer interface standard used to connect

solid-state drive A solid-state drive (SSD) is a type of solid-state storage device that uses integrated circuits to store data persistently. It is sometimes called semiconductor storage device, solid-state device, or solid-state disk. SSDs rely on non- ...

s (SSDs) to a computer. It defines the physical connector, electrical characteristics, and supported communication protocols. U.2 was developed for the enterprise storage market and is designed to support multiple types of drives, including those using

PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe, is a high-speed standard used to connect hardware components inside computers. It is designed to replace older expansion bus standards such as Peripher ...

(typically with

NVM Express NVM Express (NVMe) or Non-Volatile Memory Host Controller Interface Specification (NVMHCIS) is an open, logical-device interface functional specification, specification for accessing a computer's non-volatile storage media usually attached via th ...

), as well as SAS and

SATA SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard ...

. The interface supports up to four PCIe lanes and two SATA lanes, enabling high data transfer rates while maintaining compatibility with existing drive technologies.

History

The Enterprise SSD form factor was developed by the SSD Form Factor Working Group (SFFWG). The specification was released on December 20, 2011, as a mechanism for providing PCI Express connections to SSDs for the enterprise market. Goals included being usable in existing 2.5" and 3.5" form factors, to be hot swappable and to allow legacy SAS and SATA drives to be mixed using the same connector family. In June 2015, the SFFWG announced that the connector was being renamed to U.2.

Connector

The U.2 connector is mechanically identical to the SATA Express device plug, but provides four PCI Express lanes through a different usage of available pins. U.2 devices may be connected to an M.2 port using an adapter.

Availability

In November 2015, Intel introduced the 750 series SSD which is available in both PCI Express and U.2 variants. Since then, U.2 has achieved a high level of support from the major storage vendors and storage appliance suppliers.

U.2 compared with M.2

U.2 can use 3.3V, 5V and 12V for power, while M.2 only supports 3.3V.

As implemented

While the U.2 standard does not imply a form factor of the device that uses it, in practice U.2 is used only on 2.5" SSDs. 2.5" drives are physically larger than M.2 drives and thus typically have larger capacities.

U.3

U.3 (SFF-TA-1001) is built on the U.2 spec and uses the same SFF-8639 connector. A single "tri-mode" (PCIe/SATA/SAS) backplane receptacle can handle all three types of connections; the controller automatically detects the type of connection used. This is unlike U.2, where users need to use separate controllers for SATA/SAS and NVMe. U.3 devices are required to be backwards-compatible with U.2 hosts. U.2 devices are not compatible and cannot be used with U.3 hosts.

References

{{Authority control Computer buses Computer connectors Serial ATA Peripheral Component Interconnect SCSI Computer storage buses SATA Express