I know there are lots of threads asking about which SD cards are more suitable for RasPi, but I haven't found anything that gives a good answer. I looked at the RasPi wiki (http://elinux.org/RPi_Verified.....eripherals
) to see some verified SD cards and from there I looked at the benchmark results (http://www.sakoman.com/OMAP/mi.....sults.html
) and this is where things got interesting.
SD cards have a class rating that indicates the sequential write speed. So a class 4 card will write at 4MB/s and a class 6 card at 6MB/s. So you might naturally assume the higher the class the better… but not necessarily. SD cards are used in cameras and video cameras where they sequentially read and write large file sizes. But a computer uses random read/write access and the file sizes are typically very much smaller.
The above benchmarks indicate that the SD card is going to have a signficant impact on the overall RasPi performance. For example, one test was to install Gnome r12. The fastest SD card was a Topram 8Gb Class 4 which took 1252 seconds, an average card is the ADATA 8Gb Class 6 card which took 5363 seconds, but the slowest card was an RiData 4Gb Class 6 card which took 15707 seconds… a whopping 12 times slower than the Topram!!
So I wondered if there was a simple, consistent test that could be quickly and easily run on a Windows box that would help everyone identify more suitable cards and maybe this thread could be used as a collection pot for the various results.
The software I used was Crystal Disk Mark (http://crystalmark.info/?lang=en
). This performs a sequential test, which is good for cameras and video recorders, but it also does random access tests with record sizes of 512kb and 4kb… which is more like what the RasPi will use.
To kick things off, here are the results for two 16Gb Class 6 SD cards that I have; one from ADATA the other from Lexar.
NOTE: I set the file size to 50Mb, all other settings are default.
ADATA 16Gb Class 6
Lexar 16Gb Class 6
Both cards are the same capacity and class, but look how fast the sequential write speed is of the Lexar compared to the Adata… more than twice as fast! The Lexar is definitely the card I want in my camera.
But now look at the figures for the 512k and 4k tests. The 512k results are similar for both but while the 4k read tests are faster for the Lexar, the 4k write tests for the Lexar are terrible… 5 times slower than the Adata. So maybe the ADATA card is the one I want to use for RasPi?
If others would like to share the performance figures for their SD cards I'm sure there would be some interest.
(Those with experience may like to comment on the real world impact of these numbers).