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
something I've been thinking about a lot is engineering panic signals into embedded stuff.
put a rail supervisor on the incoming supply, and leave enough capacitance on the low side of your buck(-boost) reg to keep the device running for a few milliseconds when the power is pulled. treat the supervisor IC's PG output as a panic signal and have it trigger an IO interrupt on your MCU/SoC which tells the filesystem code to ensure integrity, flush, and halt IO.
@gsuberland That and ECC RAM.
Doesn't help that as laptops have gotten popular devs have gotten lazy at filesystem power loss support.
@gsuberland Yep this is something I've started to design into my architecture, prototyping on the trigger crossbar and then scaling to a next-generation system with more capabilities on my next project.
Luckily there's not a whole lot to worry about in terms of data integrity because MicroKVS is a) seldom written and b) designed to not corrupt data if interrupted at any point.
At some point I need to build a board where I can test that assumption: have one MCU that controls power to another and the target is just constantly booting up, doing KVS reads and writes, and being randomly reset/power cycled.
And see if I can make it break.
@apicultor what I mean is that if the goal is to get around bitflips and bitrot and write wear issues with ZFS, the correction itself will cause more write wear.
it's better than not using ZFS, but it accelerates the overall inevitability of complete failure.
@gsuberland Ah, legit, yes.
What about SD cards that don't suck — as in, ones that have their flash configured in SLC mode so you get bonkers durability like 5K TBW (for the largest ones, reducing proportionally by capacity):
https://www.transcend-info.com/embedded/product/embedded-memory-cards/usd230i
https://www.transcend-info.com/embedded/product/embedded-memory-cards/usd240i
Not quite as impressive, but still ~2.8K TBW if I did my math right (it's expressed in hours of HD video at 26 Mbit/s):
https://www.transcend-info.com/product/memory-card/usd350v
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