SD card speed?

[the wombat]

I see that the SD card needs to be at least Class 4 to work properly, but is there any benefit to going faster? Can the Pi max a Class 6? A Class 10? A Class 15?

[pluggy]

Some of the speed fiends advocate using Class 10. In the real world it won't make much odds, linux spends a lot more time reading than writing and class is all about write speed. They read a whole lot faster than write anyway.

[lb]

The class ratings are meaningless for usage in the Pi. Where did you read that class 4 is required? That is not correct.

[the wombat]

From the beginners startup guide - "A brand-name (not generic) Class 4 card of 4GB or more is recommended. " While this is not an absolute prohibition on anything smaller/slower, I take it to be that it ill-advised to do so.

[itimpi]

It should also say that anything faster than Class 10 is not going to work, and some Class 10 seem to have problems. In fact in the early days of the Pi most Class 10 cards seemed to have problems although since then the firmware has improved and they tend to be more reliable. I have also heard that there may be an upper limit of 32GB in the size of cards that can be handled but have no experience of such large cards myself.

In my experience something like a Sandisk Ultra Class 6 seem to give the best mix of reliability, performance and price (although others may beg to differ )

[TideMan]

You can check out the speed of your SD card reading, then writing using:

Code: Select all

dd if=/dev/zero of=~/test.tmp bs=500K count=1024
dd if=~/test.bmp of=/dev/null bs=500K count=1024

What I found to my surprise was:
Class 10:
Reading: 524 MB in 71.5 s = 7.3MB/s
Writing: 524 MB in 26.5 s = 19.8 MB/s
Class 4:
Reading: 524 MB in 40.99s = 12.9 MB/s
Writing: 524 MB in 25.09 s = 20.9 MB/s

So, for me, my Class 4 card is faster than the Class 10 card, both cards set up the same.

[SN]

Don't use the cheapo Aldi 8Gb cards - they're excruciatingly slow

[penguintutor]

I have tried different SD cards from cheap ones to Sandisk Ultra II / Sandisk Extreme III

There may be a slight performance difference, but looking at startup and general useability there was no noticeable difference in actual usable performance.

That's not to say there isn't any benefit in faster cards, particularly if you are reading / writing a lot to the card, but that the difference in general use is not going to make a huge difference.