http://blog.tdg5.com/tuning-dd-block-size/
As an example of the impact that an inefficient/optimal block size can have, I’ve run a few tests for your consideration. These results are all specific to my hardware, and though they may offer a rule-of-thumb for similar situations, it’s important to keep in mind that there is no universally correct block size; what is optimal for one situation may be terribly inefficient for another.
