What's the max speed for the SD slot?


11 posts
by adi518 » Fri Jun 22, 2012 10:55 pm
I did some google queries but none gave me a distinct answer.

I'm contemplating whether I should a Sandisk Extreme III (45mb/s) or go with the slower versions, namely 30mb/s ultra edition cards.

What say you? :geek:
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by dom » Sat Jun 23, 2012 12:24 am
With latest firmware, 50MHz clock of 4bits data, so 25MB/s maximum.
There's overhead so you won't reach that, but I've seen 22MB/s.

However a lot of the very high speed cards have very poor random access performance,
so in real use are slower than, say, a class 4 Sandisk Ultra.
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by adi518 » Sat Jun 23, 2012 12:54 am
I see. So maybe I should go for the lower end Ultra cards? They supposedly run @ 30mb/s, I guess that should save some cost and still allow maximum boot speed. But I'm thinking, maybe there's a possibility that a future firmware update will allow the controller to go faster than 25mb/s?
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by lb » Sat Jun 23, 2012 1:58 am
The controller should be able to go faster in theory (with UHS-I cards), but the electrical specifications of the Pi are not compatible to UHS-I - 3.3V signaling only. So it won't get any faster than about 22 MB/s in practice.

As dom already said, it's much more important that the SD card is reasonably fast at random writes. That's the primary bottleneck, and most cards, even fast and expensive ones, are pretty bad at it.
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by JeremyF » Sat Jun 23, 2012 2:00 am
A USB 2.0 flash drive may end up faster, and more reliable
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by adi518 » Sat Jun 23, 2012 2:16 am
lb wrote:The controller should be able to go faster in theory (with UHS-I cards), but the electrical specifications of the Pi are not compatible to UHS-I - 3.3V signaling only. So it won't get any faster than about 22 MB/s in practice.

As dom already said, it's much more important that the SD card is reasonably fast at random writes. That's the primary bottleneck, and most cards, even fast and expensive ones, are pretty bad at it.


So what cards are known to have fast random writes?

I don't want to use a usb flash drive, they're too bulky. I want to make a minimal Pi setup.
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by itimpi » Sat Jun 23, 2012 5:33 am
adi518 wrote:I don't want to use a usb flash drive, they're too bulky. I want to make a minimal Pi setup.

I have been thinking of acquiring one of the micro USB flash drives like this one to keep size down. Anyone know how they perform in the Pi?
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by adi518 » Sat Jun 23, 2012 2:17 pm
itimpi wrote:
adi518 wrote:I don't want to use a usb flash drive, they're too bulky. I want to make a minimal Pi setup.

I have been thinking of acquiring one of the micro USB flash drives like this one to keep size down. Anyone know how they perform in the Pi?


holy, that's really a small flash drive. I guess I meant for something like this to set an alternative. I think it will work. Question is, how fast it is? I already ran into seriously slow doks before.

The only downside is the fact it's gonna hog a usb slot while you could use the slot for other things like a wifi dongle and/or a wireless keyboard dongle. It's something to think about.
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by adi518 » Sat Jun 30, 2012 12:30 am
bump... I need some more advice! Any cards known to have good random-access speeds?
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by lb » Sat Jun 30, 2012 2:42 am
Check out the SD card benchmark thread here: viewtopic.php?f=2&t=4076
The results should translate over to the Pi. Most cards perform, really, really badly, with just about 10 IOPS per second. On the other hand, some cards are capable of hundreds of IOPS per second, and actually get into more than a MB/s at random writes! Try to get one of those SD cards, but beware - sometimes manufacturers switch controllers and whatnot, and performance characteristics aren't consistent for a given SD card model.

The people from Linaro have also done some nice research: https://wiki.linaro.org/WorkingGroups/K ... CardSurvey
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by adi518 » Sat Jun 30, 2012 10:50 pm
Those links were great, thanks!

Ok, I think I'm gonna go with a Sandisk Ultra 30MB/s Class 6. Seems to have good all around performance. Thing is, I can't seem to obtain a Class 6 version of this card from any of the nearby stores. Those only have the Class 10 version so I'm not sure which should I go for. I can get a Class 6 one from ebay. I know it's not the most reliable source for genuine goods like this but some sellers seem legit with a somewhat close to the nominal price of the local stores.
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