SD Express gives removable storage speeds a leap forward

Nobody can deny that SD and microSD cards are useful, but I groan every time I have to use one. Their performance starts at “abysmal” and tops out at “mediocre.” That might change soon. The SD Association just ratified version 7.0 of the SD standard, and included in this revision is a new type of SD card interface called SD Express. Yep, you guessed it—it's based on NVM Express. On top of the potentially higher-speed interface, SD 7.0 pushes the maximum capacity of SD cards to 128 TB with the SD UC standard. That capacity should be good far into the future.

SD Express makes use of the second row of pins already included on Ultra High Speed (UHS) SD cards to hook up a PCIe 3.0 x1 connection. We desktop users might sneer at a single PCIe lane, but PCIe 3.0 tops out at 985 MB/s bidirectionally. That's a solid improvement over the 624 MB/sec peak of rare and expensive UHS-III cards, and a great leap beyond the typical performance of existing removable storage (SD cards or otherwise). It comes without having to revise the form factor of the cards, too.

The NVMe protocol allows each CPU core to have a dedicated command queue. That should drastically improve storage performance for many-core mobile SoCs. Besides the better performance, the PCIe interface brings bus mastering support. That means the CPU doesn't have to be directly involved when accessing the card, which in turn could end up reducing power draw for the new cards. That may offset the increased power usage of the SoC as it no longer has to wait on the storage all the time. 

Of course, you'll need new hardware to support the new SD cards, although the cards are backward compatible with the standard SD UHS-I interface. If you're thinking that this turns SD cards into something like a removable SSD, that's what the SD Association wants you to think. The group's video even uses those exact terms. Hopefully the new tech finds its way into every space we usually see SD cards sooner rather than later. If you're curious about the technical details, you can check out the SD Association's white paper

Comments closed
    • Takeshi7
    • 1 year ago

    Now I hope Intel/Micron start making 3DXPoint SD and CompactFlash cards.

      • willmore
      • 1 year ago

      Why? Do you need more expensive and lower capacity memory cards for some reason?

    • NTMBK
    • 1 year ago

    Ah, I see they’ve taken a page from the USB-C book. “Let’s make one physical interface actually correspond to multiple different incompatible standards! What could possibly be confusing about that!”

      • RAGEPRO
      • 1 year ago

      SD Express cards will be compatible with both SD Express and devices that expect a UHS SD card.

        • just brew it!
        • 1 year ago

        …but will be limited to UHS-I speeds in older devices even if the host is UHS-II or -III.

          • GTVic
          • 1 year ago

          If you look at newegg there are only 13 cards for sale that use UHS-II and none that use UHS-III. So maybe they are ditching a standard that will never see widespread use.

            • just brew it!
            • 1 year ago

            That’s entirely possible…

      • synthtel2
      • 1 year ago

      Bog-standard SD cards have already got both SPI and their own thing. They’ve been dealing with this for ages; making it as fast as possible might get weird, but I trust it’ll at least be usable everywhere.

    • djayjp
    • 1 year ago

    The sequential speed or raw bandwidth isn’t the issue, but rather the random performance. I don’t see how this addresses that.

      • RAGEPRO
      • 1 year ago

      The NVMe protocol is designed for maximum random-access performance. It supports non-locking multi-core storage access.

        • Waco
        • 1 year ago

        This. The entire design is to reduce CPU usage and latency.

        • djayjp
        • 1 year ago

        Ah I see, thank you. Though I guess we won’t know for sure until benchmarks!

      • just brew it!
      • 1 year ago

      That’s up to the designer of the flash controller in the memory card. So it’ll probably be pretty good on “premium” cards, and suck on cheaper ones.

      IOW, the interface is not going to be the bottleneck here.

    • meerkt
    • 1 year ago

    The image implies no Micro-sized for SD Express?

      • willmore
      • 1 year ago

      That was going to be my question as well. Considering more uSD cards are used in cell phones and other applications than full sized SD cards are, it’s way more important to me to know how they’ll be improved. Being smaller, they should be easier to control the impedance of the signal lines which should make them easier to implement. Also, I wonder if there was ever a uSD to SD adapter for those pins? I never bought a UHS-2 or better card, so I’ve never run into that situation.

        • UberGerbil
        • 1 year ago

        Cell phones and other small consumer devices certainly mean that µSD is more common, but they don’t have the most pressing need for higher bandwidth. That’s the domain of SD — eg 8K video in DSLRs. That’s also the market that will pay for this kind of bleeding edge stuff. I don’t see any technical reason why it won’t filter down to µSD eventually. IOW, µSD may be way more important to you but you aren’t all that important to the SD Association.

        Sure it would be nice if µSD cards were faster right now — but as often as not they’re not constrained by the interface anyway: they use slow flash with minimal parallelism and pokey (but cheap) controllers.

      • RAGEPRO
      • 1 year ago

      [s<]The PDF does show an example microSD Express card.[/s<] I was mistaken, read below.

        • just brew it!
        • 1 year ago

        The only place I see a microSD card is the generic image on page 5 illustrating how they are “merging” SD and PCIe/NVMe tech. The pinout description on page 6 only shows a full-size card.

        Doesn’t mean microSD form factor won’t happen, but I’d take it as a hint that it will roll out in full-size SD form factor first.

        Space-constrained mobile devices don’t need that kind of bandwidth anyway. At least, not yet (or any time soon).

          • willmore
          • 1 year ago

          You mean cell phones and single board computers? The latter certainly feel differently.

          • RAGEPRO
          • 1 year ago

          Actually, I was referring to the other PDF (linked on “just ratified” above) … which is showing a microSD as an example of an SDUC card, not an SD Express card. So yeah, maybe it isn’t coming to the microSD format. Would be a shame.

    • DPete27
    • 1 year ago

    Was the second row of pins completely unused previously? How were they so forward-thinking to add that row 4(?) years ago and they’re just now starting to use it?

      • Goty
      • 1 year ago

      They’ve been there since the introduction of the UHS-II and UHS-III interfaces, but I think they never saw much use outside of things like high-end DSLRs.

Pin It on Pinterest

Share This