Graham Sutherland / PolynomialMay 26, 2024

SD cards are the literal worst.

they've expanded to be the size of small hard drives, and devices like the rpi keep using them as boot media, but they:

- use garbage tier low endurance flash cells internally
- have little to no overprovisioning for wear
- perform only the most basic wear levelling
- have no protocol level integrity checking
- have few internal error correction features, if any
- decay comparatively quickly without patrol scrubs
- do not perform patrol scrubs
- cannot do PLP

Show threadIgnas KielaMay 26, 2024
@gsuberland what's really annoying is that they could do a lot of that, but don't because it's marginally cheaper and for most consumers, it's difficult to notice the regularity of their failures.

eMMC is essentially the same stuff but soldered on, but because it's always used in a large scale, companies will notice all of that stuff, and therefore money will be spent on reliability.

realistically, the only thing SD cards couldn't do is PLP
Show threadZimmieMay 27, 2024

@ignaloidas @gsuberland SD and miniSD cards *could* do PLP. Just use a microSD card internally and fill the rest of the space with capacitors.

I know some microSD cards use a proper SSD controller with a single channel to a flash die. They have real wear leveling and page sparing. Not sure if they do internal patrol scrubs. SanDisk Industrial is one of the lines which works this way.

Show threadIgnas KielaMay 27, 2024
@bob_zim @gsuberland I don't think that kind of PLP is in any way cost efficient, just some helping from host side would be cheaper.

And yeah, you can get proper SD cards if you go for industrial ones, but it's both significantly more expensive, and harder to get. Even in SBC-focused shops, rarely will you see one (even though you'd really want one for SBCs).
Show threadZimmieMay 27, 2024

@ignaloidas @gsuberland It would be exactly as cost (in)efficient as it is with desktop SSDs, since that’s how they do PLP.

As for the added expensive of high-quality cards, basically every card of a given form factor from 64 GB down is the same price: about $10 US. I was recently shopping for a pile of cards. The most expensive I found were $12.70 for single industrial 16 GB cards. Oddly enough, 32 GB (the max capacity for SDHC) industrial cards were $11.

For 128 GB and higher, yes, high-quality cards start getting more expensive than the low-quality cards. I personally don’t use SD for anything except data which is trivial to reconstruct (like maps in a GPSr), or functionally disposable (like dashcam footage).

Show threadIgnas Kiela
@bob_zim @gsuberland eh, there is a fair difference - to fit enough capacitance in SD card, you need extremely high density capacitors, while desktop SSD's can allow themselves to use cheaper, less dense capacitors (or capacitors with larger minimum sizes, like polymer or tantalum or aluminum)

For the price, the last time I looked it would cost significantly more for me to get at hobbyist amounts (<10), partly because at least from my looking, the prices were noticeably higher, partly because I would need to deal with international shipping because there's no big suppliers local to me. I could get the same capacity but crappier cards faster and ~2x cheaper just getting from local stores.
May 27, 2024 at 9:04pmWeb
Show threadZimmieMay 27, 2024

@ignaloidas @gsuberland An SD card only drives the controller and one die. It doesn’t need nearly as much power to complete in-flight writes. M.2 drives (e.g, Micron 7450) already typically have less volume in capacitors than the spare volume in an SD card.

Cost of shipping can definitely be a pain.