SD Card Speed Ratings

I am pondering buying several micro SD cards for cross use among Nano and Xavier NX. UHS-I is a fairly obvious requirement. Some other speeds are “compatible”, but will throttle back to slower speeds. What I am wondering is if all UHS-I cards will work at their rated speeds regardless being UHS-I/u1 class 10 80MB/sec, or being UHS-1/u1 class 10 100MB/s, or being UHS-1/u3 A2 160MB/s?

Any of these should be “compatible” and work, but my question is whether paying for the extra speed of going from 80MB/s to 100MB/s or to 160MB/s will actually be useful? For all I know the faster bandwidths could work or might just be ignored and cost more. I am interested in general what speed ratings will actually help within micro SD cards having a UHS-I rating.

It really depends on workload, more specifically what kind of write speeds you’ll need. A class 10/UHC-1/V10 card only has to have a minimum write speed of 10MB/s. That’s pretty slow to be running an operating system on. I’d say a UHS-3/V30 card would be the minimum. Of course they can have sequential write speeds above the minimum required for the class but if the card specs don’t specify write speed (most don’t), all you have to go on is the class. For instance, I have a few older Samsung Pro 64gb cards that are rated at UHS-3 but will happily do 80MB/s write speed.

If you’re going to stick with UHS-1, and the normal workload is going to be heavily read oriented, then I’d stick with 100MB/s or lower unless you can find a good deal. In a heavily read-oriented workload, much of the data is going to come from the filesystem cache anyway.

AFAIK, the UHS-II card on Nano would fallback to UHS-I speed class.

According to the driver code and design guide, I guess this should happen on NX too because there is no FD156 or HD312 here.


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