Someone at the SDcard Association lately is feeling the need for speed, and considering our stance on SD cards we’re glad to see it. The association has announced a new bus in the UHS-III specification as well as new A2 performance label for SD cards.
What is UHS?
The UHS bus defines two different things. First, it defines the maximum theoretical speed of the controller in a device, and it also specifies the maximum speed of a card itself separately from its performance rating. A card being rated for a specific bus isn’t a promise that cards are capable of reaching that, but a maximum speed at which a reader or device can utilize a card at. SD cards and readers are generally both forward and backward comparable outside of edge cases (usually related to size). UHS-III is a clean doubling of the maximum rate UHS-II was capable of.
About those SD card speed ratings
There are at this point no less than five different ways to indicate how fast an SD card is. The quickest way to get a look at those — and the new A2 rating — is with a quick chart.
Notice I said five earlier and only included four ratings in the chart? That’s because the fifth is a bit arbitrary. Some manufacturers still ship cards with a ###x rating. This is an old measurement based on the read speeds of a CD-ROM drive. Although more information is never a bad thing, as the SD association doesn’t control x ratings they don’t equate to anything else in the chart. These ratings are frequently mixed. An excellent example of this being the card currently in my phone which carries C10, U3, and 1800x ratings. The application speed class ratings are the two newest and hardest to quantify. The other ratings call for a minimum sequential write speed as their requirement. A1 and now A2 call for a minimum sustained number of IOPS.
What are IOPS?
IOPS is an acronym for input/output operations per second. IOPS are a lot simpler than they sound simply being the number of transfers per second a device can handle. They do need a size applied to them, which the SD association has neglected to include. However, typically random performance is rated at a 4k block size which is what they’re using for the A1 and A2 ratings.
A1 and now A2
The A2 application performance class is new and A1 isn’t exactly old as it was announced last fall. In fact, very few A1 cards have even made it to market yet. A1 calls for 1500 read and 500 write random IOPS, which are not high bars and any good card should hit this anyway. A2 is a bit more demanding, calling for 4000 read and a much more staggering 2000 random write IOPS. A2 also adds several new requirements controller side for cards such as command queuing (a common feature on SSD’s). It also introduces self-maintenance (likely similar to TRIM) and a new cache function. The cache function is the most convening of these we’ll likely see cheaper cards utilize it to meet the class requirement but perform inconsistently once their cache is exhausted.
What’s going to use these?
These high speeds look impressive on paper but need hardware to go with them. I don’t expect phones to be first devices to use these cards. The past three generations of flagships devices have been limited to the UHS-I bus. This includes last years Pixel, the HTC 10, and the Galaxy S7. This year’s flagships are going to be the same story as well. The specifications for the new SD835 processor, which is so new it hasn’t made it to anyone to test yet, still only call for UHS-I capabilities. With demands for things like 4k video and phones offering larger RAW photo capabilities, it’s only a matter of time phones adopt the faster bus speed.
Furthermore, devices like drones and action cameras using SD cards as their primary storage will likely adopt UHS-III quickly to keep up with ever increasing data storage requirements.
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