People often misunderstand the speed ratings of SD cards. They are built cheaply, and historically have primarily targeted digital cameras. That meant they emphasized sequential IO performance, because that’s how pictures and video are handled. When we plug an SD card into a Raspberry Pi, however, most of our applications will use random reads and writes, which is often abysmal on these cards.
Enter the A1 and A2 speed ratings. These were created by the SD Card Association to guarantee a minimum random IO performance, which is exactly what we want for single board computers.
There’s been a blog post floating around about how the A2 class is marketing BS. In some cases, I’ve seen this cited as evidence of the A1 class also being marketing BS. After all, the top cards on the chart for random IO, the Samsung Evo+ and Pro+, don’t have any A class marks on them.
This misunderstands what these marks are for and how companies make their cards. Your card has to meet a minimum speed to have a given mark. You are free to exceed it. It just so happens that Samsung makes some really good cards and have yet to apply for the mark on them.
Samsung could change the underlying parts to one that still meets its certification marks for the model, but with far worse random IO performance. I don’t think Samsung would do that, but they would technically be within their rights to do so. This kind of thing has happened in the storage industry before. Just this past year, Western Digital put Shingled Magnetic Recording on NAS drives (SMR is a hard drive tech that craters random write performance, making them completely unsuitable for NAS usage). XPG swapped out the controller on the SX8200 Pro to a cheaper, slower one invaliding the praiseworthy reviews they got at launch.
Even if Samsung wouldn’t do that, it’s still something where you have to go out of your way to find the best SD card. Well-informed consumers will do that, but the Raspberry Pi serves a broad market. Remember, its original purpose was education. You can’t expect people to even know what to research when they’re just starting out. It also doesn’t help that major review sites don’t touch on SD cards. Benchmarking can be a tricky thing to do right, and most hobbyist bloggers don’t have the resources to do a good, controlled test, even if they mean well.
What the A class marks do is give a clear indication of meeting a certain minimum standard, at least in theory. Independent reviews are always good in order to keep everyone honest, but if you don’t have time to look at them, you can grab an A1 card and put it in your Pi and you’ll be fine.
As the blog post noted above states, A2 cards don’t always live up to their specs. According to a followup post, this appears to be due to things the OS kernel needs to support, rather than the card itself. It’s also possible for a company to try to pull a fast one, where they launch with a card that meets spec, and then quietly change it. However, if they do, they don’t just have consumer backlash to contend with, but the SD Card Association lawyers. Since they hold the trademark on the A class marks, they have the right to sue someone who is misusing it.
Marketing isn’t just about making people into mindless consumers. It can also be about conveying correct information about your product. That’s what the A classes are intended to do. Nobody knew that Samsung Evo and Pro cards were good until somebody tested them independently. With the A class marks, we have at least some kind of promise backed up with legal implications for breaking it.