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I'm testing a pair of sd cards this way:

The first card I did this on (sandisk class 4) gave a write speed of 5.4 MB/s, which is about what I expected based on the results on that page. However, the read speed was 21.6 MB/s, which there are almost no class 4 cards listed that come anywhere close.

The other card (adata class 10) had a (disappointing) write speed of 7.6 MB/s, and an identical read speed, 21.6 MB/s.

I realized the file cache could probably interfere with this, even though that entire file will not fit into the available RAM. So I re-ran the tests, flushing the cache after the write test and before the read test:

echo -n 3 > /proc/sys/vm/drop_caches

Output from free demonstrates this is effective.

However, again both read tests are 21.6 MB/s. I've tried rebooting -- exact same thing. The cards contain identical raspbian systems with the stock 3.2.27+ kernel.

Anyone know what is up with that?

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3 Answers 3

up vote 3 down vote accepted

Well structured question. Try using:

hdparm -t /dev/sdb

Assuming your device is on /dev/sdb. Also, perhaps your devices just have equal read speeds? Do you have any others to test out?

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hdparm cannot be used for anything in this regard as the sd card reader in the pi is not an ide or sata device. – goldilocks Jan 23 '13 at 22:04
Huh -- ok then! Anyway, the hdparm test comes out the same as the dd test. – goldilocks Jan 24 '13 at 3:00

It's very likely that the RPi itself it bottlenecking the SD card, making it irrelevant what the speed of the card is, if it can't interface it fast enough.

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There are several factors to consider, not just card speed. As one person as stated, the Raspberry Pi could be the bottle neck, yet that could be its hardware, or software driver. Be advised, I'm using my Raspberry Pi ONLY to learn Mathematica; my advice on this comes from an electronics background and extensive IT support over 3 decades. I also looked at the big chart of cards and their performance. I ignore most brands bar SanDisk. This is due to good experience, and frankly they're most readily available to me. The clear picture I got was that UHS-1 cards are faster, so consider them. Interestingly, I didn't expect to see SDXC cards on the list, SDXC and cheap hardware are usually mutually exclusive.

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I wasn't aware of it when I wrote this question, but as it turns out the Pi's SD card reader has a maximum transfer rate of 25 MB/s, which is why you don't see anyone reporting much over 20 regardless of the card. – goldilocks Oct 15 at 0:07

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