Following these instructions: http://elinux.org/RPi_SD_cards#Performance
I've got a 4GB SD card with Raspbian Wheezy.
sync; time dd if=/dev/zero of=~/test.tmp bs=500K count=1024; time sync gave me an error due to insufficent storage and reported a very low write speed (~2 MB/s)
I decreased the count to 128 ad then the write speed shot up to 34.4 MB/s.
Regardless of the count, when reading (with
dd if=~/test.tmp of=/dev/null bs=500K count=1024)
dd reports are read speed of 171 MB/s.
Is that normal? Everything I saw online seemed to suggest the SD Port topped out around 20 MB/s.
EDIT: As per @user2813274's comment, /dev/null and /dev/zero are special cases.
When I tried dd'ing a normal file with
dd if=~/my_file of=~/test.tmp bs=500k count=128 dd reported a transfer speed of 31.0 MB/s.
31.0 MB/s is still substantially larger than the apparent maximum of 23.3 MB/s (link above) on a SanDisk Extreme III Class 10 SDHC card. I'm using the SD card that came with the Pi.
But, when I remove the
count=1024 flag for
dd copies the entire file, instead of just a portion of it, and then it reports a variable transfer speed between 5.0 - 7.0 MB/s.