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

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