SD and microSD cards performance


remjey

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Hello, I’m getting a Pyra and I had some questions about SD cards. I did a little research and it seams fast SD cards have extremely bad random access performance (like +80Mo/s seq rw and ~2Mo/s random rw). Also there are problems with destructive readings and, apparently, problems with UHS cards that physically wear faster when used in non UHS certified ports.

Supposing I’m ready to pay and that I don’t need lots of space (16-32Go), do you know if I can find a (micro)SD card that will sustain good performance in seq and random access for a long time? Kind of like “industrial” ComactFlash but for SD cards.

Thank you!

PS: I know about F2FS and other pure-journal FS that make random write faster, but random read stays weak afaik.

TLDR: what is the most durable, fast SD or microSD card I can put inside a Pyra?
 
I did a little research and it seams fast SD cards have extremely bad random access performance (like +80Mo/s seq rw and ~2Mo/s random rw).
I can confirm that. "Fast" SD cards usually implement an MLC/TLC RAID, which boosts sequential transfer speeds, but reduces random transfers speeds because it always requires to r/w more flash cells than would be needed.

Also there are problems with destructive readings and, apparently, problems with UHS cards that physically wear faster when used in non UHS certified ports.
Never heard of it, but that might just be lack of knowledge on my side.
Write operations on flash media are destructive in the long run, because they oxidate the flash cell's envelopes which make them persistent. This should not happen during pure read access, but after all nobody knows what happens inside a flash controller.

Supposing I’m ready to pay and that I don’t need lots of space (16-32Go), do you know if I can find a (micro)SD card that will sustain good performance in seq and random access for a long time? Kind of like “industrial” ComactFlash but for SD cards.
Apparently there are "industrial" µSD cards. [1][2][3] (just some random examples)
It seems to me that up to 8GB are available with SLC flash, which would be optimal if you want to run an OS from it. A moderately lean Debian system can fit in 8GB with no problems. Above 8GB there are only MLC cards, which should still be ok if you need more space.

I know none of those cards I linked. Unfortunately they don't provide data for random access. I don't have numbers right now, but my fastest µSD card in terms of random access is an 8GB Transcend card that I bough at a time when 32GB were just entering the market.
Maybe someone with more experience/knowledge than me can recommend one or two "industrial" µSD cards starting from 8GB with known (tested) good random access transfer, that Evil Dragon could ultimately sell in his shop along with the Pyra.


[1] https://www.sandisk.com/oem-design/industrial/industrial-cards
[2] http://www.digikey.de/en/product-highlight/a/atp/industrial-grade-microsd-microsdhc-cards
[3] http://www.kingston.com/de/flash/microsd_cards/sdcit
 
You cant have booth, speed and reliability, at the same time.
 
I've perhaps been lucky, but I've not had a failure from a Sandisc Ultra or Extreme card except for one that I formatted wrongly and somehow managed to fritz completely, such that most machines don't even recognise a card is inserted.

But those are marketed by their serial speed performance. They seem adequately fast for random access, but I've not tested that specifically. When it comes to reliability, I don't believe you can trust these things ever absolutely, so a backup strategy becomes critical.
 
You cant have booth, speed and reliability, at the same time.
Actually, you can.
More precisely: You can only either get both or none.

Compared to MLC or TLC, which is used in consumer SD cards, SLC flash is both reliable and fast (in random access scenarios).
However, it is neither cheap nor does it allow for high storage density. This is why you only get 8GB in µSD format and it comes at a hefty USD 100 for a single card. [1] You could reduce the cost to USD 80 per unit if you order a hundred pieces, hence the idea to have ED (or someone else for that matter) as a central hub. But if I were ED, I'd only do that if I had more detailed technical info about these cards than the manufacturer currently provides.


[1] http://www.digikey.de/product-detail/en/atp-electronics-inc/AF8GUDI-OEM/AF8GUDI-OEM-ND/4271271
 
Good point sulu,
But the prices are whoa.
I always assumed single state cells only cost about two or three times of normal MLC.
 
I don't know what gave you that impression. You're basically going back to the old technology that was used when SD cards topped out at under a GB I think. Improvements in yields have made some headway in reducing prices, but for SLC cells you need your 8 billlion flash cells to work, and there's not the same economy of scale operating in these parts (although they may be getting some benefit from those 8 billion cell parts being usable in 16GB MLC cards, or 32GB TLC ones, I dunno).
 
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