U.2

SSDs with U.2 interface

U.2, formerly known as SFF-8639, is a computer interface for connecting SSDs to a computer. It uses up to four PCI Express lanes.

It is developed for the enterprise market and designed to support new PCI Express drives along with SAS and SATA drives.

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 supporting existing 2.5" and 3.5" mechanical enclosures, to be hot swappable and to allow legacy SAS and SATA drives to be mixed using the same connector family.^[1]

In June 2015 the SFFWG announced that the connector was being renamed to U.2.^[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.^[3][4]

U.2 devices may be connected to an M.2 port using an adapter.^[5]

Availability

In November 2015, Intel introduced their 750 series which came in both PCI Express and U.2 variants.^[6]

Benefits of U.2 over M.2

• U.2 supports hot-swap^[7]and M.2 does not^[8].
• U.2 drives incorporate an enclosure and M.2 drives are typically bare circuit boards and chips.
  • The U.2 drive enclosure protects the internal chips and circuitry from accidental damage from scratches, electrical shorting, and electrostatic discharge. A person's hand can transfer ESD to the M.2 drive during installation, and a metal tool could bump the M.2 drive during or after installation.
  • The smooth surface of a U.2 drive enclosure is less likely to catch dust than the bumpy surface of an M.2 drive.
• U.2 drives connect remotely to the motherboard via a flexible cable, while M.2 is attached directly and rigidly.
  • U.2 drives can be installed anywhere in the case where there is room, while M.2 drives must be installed exactly where the slots are on the motherboard.
  • Since U.2 drives can be installed in any of a number of places, the user can prioritize places with the best airflow, easiest access, or the most visually-appealing location.
  • Motherboards often have M.2 slots in positions that conflict with large graphics cards, which would require the user to leave empty either that M.2 slot or that PCI-Express slot. With U.2, it is easy to route the cable around graphics cards and other components so that you can use all of the U.2 slots without compromising other components.
  • Due to the rigid connection of the M.2 drives, both the M.2 drive and the connector are at higher risk of being accidentally damaged by a perpendicular force being applied to the M.2 drive sticking out, such as may happen when a person's hand bumps into the M.2 drive as they are working inside the case. Bumping into a flexible U.2 cable is unlikely to result in damage.
  • A U.2 slot with a U.2 cable takes up less space near the motherboard than an M.2 slot with an M.2 drive installed. If a desktop computer requires many drives, it would be easier for the motherboard to fit many U.2 slots than many M.2 slots.
  • Since the U.2 drive is remote to the motherboard, the user can more easily see the surface of their motherboard than if a M.2 drive were installed flatly above the motherboard. This may make it easier to diagnose problems with the motherboard such as swollen capacitors or melted chips. In rare cases, the M.2 drive may cover components that the user may want to access such as connectors, the BIOS battery, or jumpers, requiring the user to remove the M.2 drive before being able to access the blocked components.
• U.2 drives are easier to keep cool than an M.2 drive
  • Most U.2 drives use their incorporated enclosure as a large heatsink^[9] but most M.2 drives come with no heatsink^[10].
  • U.2 drives can be installed in a location inside the case with good direct air flow, such as in a front drive bay which typically has its own dedicated fans, but M.2 drives allow no flexibility in where they are installed because they must attach exactly where the motherboard manufacturer has located the M.2 slots. Since motherboards and cases may be manufactured by different companies, the locations of the M.2 slots on the motherboard typically have no relationship to the locations of the cooling fans in any particular computer case.
  • M.2 drives are closer to components that create a lot of heat, such as graphics cards and the processor.
• U.2 drives are able to sustain higher speeds for a longer duration than M.2 drives due to earlier thermal throttling of M.2 drives^[11].
  • Since M.2 drives typically have no heatsink and are not able to be positioned in a location with ideal airflow, they are more likely to suffer from thermal throttling than a U.2 drive ^[11].
  • While a drive is suffering from thermal throttling, which is necessary to protect the drive from damage, it slows down the drive's performance. This means the drive is no longer providing its advertised speed. Some M.2 drives may only able to provide their advertised speeds for brief durations before thermal throttling slows them down^[11].
• U.2 drives can have higher storage capacities than M.2 drives.
  • Since U.2 drives are physically larger than M.2 drives, it is easier for the manufacturer to place more flash storage chips on them^[12].

References