Originally Posted by
NRG
Thanks Andre but I think you've misread it, easy to do....
The memory of a card is divided into minimum memory units. The device writes data onto memory units where no data is already stored. As available memory becomes divided into smaller units through normal use, this leads to an increase in non-linear, or fragmented storage. The amount of fragmentation can reduce write speeds, so faster SD memory card speed standards help compensate for fragmentation.
What they are saying is that during normal file system usage as file are deleted and others written fragmentation occurs which can affect the write performance (by adding write latency) of the flash memory. The higher classes of card offset this increased write latency by being faster in the first place so the effect is less noticeable.
On another note these classifications of speed class are for sequential continuous writes. When an SD card is used in a computer environment, like on our RPi's the card is subjected to a small block random read and write pattern, not sequential write (well only at the time of creating the image) so the most important parameter for the card and it's not often if at all published is the 4K block random read/write performance.
There is a thread over on the RPi forum where this is explained and a whole bunch of performance results posted on numerous different SD cards. It may surprise you that some of the fastest performing cards in this 4K random test are Class4 cards and not Class10... :)