There is a chapter in the documentation about data patterns.
As far as decreasing iops: likely caused by different cache hit ratios obtained.
Thanks a lot. Normally how long time I should run the test to get a steady state value.
That is a difficult question to give a straight answer to. With storage subsystems having terabytes of RAM and multiple terabytes of SSD the only way to determine steady state is to just run it and see what happens. That is why the detailed second-by-second reporting done by Vdbench is so important. Once you know how long the warmup takes, you can use the warmup= parameter,