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

http://elinux.org/RPi_SD_cards#Performance

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:

sync
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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2 Answers 2

up vote 2 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
1  
    
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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