As SATA Express never took off because of the two-lane limitation, the SSD and motherboard industries have been looking for an alternative connector for connecting 2.5" SSDs over PCIe. SFF-8639, which is essentially SATA Express on steroids with support for four PCIe lanes, has been viewed as the most potent connector because it already has industry support in the enterprise space and with the SSD 750 Intel brought the SFF-8639 connector to the client side. Given that SFF-8639 isn't a very consumer facing name (even I've had trouble remembering the numbers), the SSD Form Factor Working Group has decided to rename the connector as U.2 to make it more marketable. That coincides well with M.2 that has already been used in the industry for a couple of years and more importantly both connectors now carry alike naming.

Whether U.2 and 2.5" PCIe SSDs take off in the client space remains to be seen, though. The biggest hurdle is the expensive cabling because unlike normal SATA and SATA Express cables, the U.2 cable consists of several small, shielded cables that increase the cost. From what I have heard that is the reason why the industry came up with SATA Express in the first place because the OEMs wanted to keep the cabling cost equivalent to existing SATA cables.

I believe M.2 will be the main connector / form factor in the client space, but there is still a market for high performance and capacity 2.5" PCIe SSDs as M.2 has more physical limitations that restrict the capacities and thermals. I can see U.2 in high-end motherboards where the connector and cable costs aren't that big of an issue, but we'll see what happens over the next year or two.

Source: Hardwarezone via PC Perspective

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  • fokka - Wednesday, June 10, 2015 - link

    it's kinda funny that thunderbolt 3 will soon enable similar speeds plus power over a much more convenient usb type-c connector, while inside the case we continue to use ugly and huge abominations like this. even m.2 is a smaller connector than this, why not simply make cables for that?

    the second i saw first images of sata express i wondered what idiot came up with that idea and u.2 hardly seems like the improvement i would wish for.
  • bernstein - Wednesday, June 10, 2015 - link

    +1 but:
    - backwards compatibility: solder a U.2 receptable onto your mainboard and the consumer can:
    - connect any sata drive
    - any sata express drive
    - M.2 has 66 pins = huge/expensive cable
    - thunderbolt is a proprietary standard from intel

    however i doubt U.2 will ever truly take off in consumer space:
    - SSDs price is essentially a function of die mm2 price, so the consumer price points will always fit onto a M.2 ssd. (and for NVMe ssds a pcie add-in board with more M.2 slots only supporting pcie is cheap)
    - SATA3 will be fast enough for a very long time for high capacity HDDs
    - which leaves hybrid drives... which are the only ones who would truly benefit from U.2, but then these only make sense in very limited scenarios...
  • TelstarTOS - Monday, June 15, 2015 - link

    I'm optimistic on this connector. My next SSD will have one (waiting for Samsung NVmE reply to Intel 750). So much better to have a standard 2,5" drive than M.2.
  • Senti - Wednesday, June 10, 2015 - link

    Well, the problem is cost. Of the controller. TB3 is going to add a noticeable bit to price tag and you won't get anywhere near 6+ ports that motherboards now love to claim (not that everyone needs those 6+ ports especially if the devices can be easily connected/disconnected).
  • Metaluna - Wednesday, June 10, 2015 - link

    For one thing, I don't think either m.2 or USB-C will work that well for hot swap trays, which are important for servers.
  • nils_ - Thursday, June 11, 2015 - link

    Another issue with Type C is that for an internal connection you want a connector that locks in place at least a bit so you can't disconnect it by accident. And I suppose cables that support this speed aren't cheap either, current ThunderBolt cables are already somewhat expensive.
  • close - Thursday, June 11, 2015 - link

    Just look at the USB 3.0 type B connector to see what backwards compatibility does to a connector :).
  • Senti - Wednesday, June 10, 2015 - link

    That should've being the next SATA standard from the start as SATAExpress is definitely not worth switching from regular SATA.
  • Maltz - Wednesday, June 10, 2015 - link

    Oh c'mon, ANOTHER one?? lol
  • Drumsticks - Wednesday, June 10, 2015 - link

    To be fair, this one already existed!

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