The Writes performances problem comming from one defective Samsung SSD 840Pro. The system don't see anything wrong.
After removed all three drives, i checked alone on a w7 PC with Samsung Magician and see nothing too, but the writes performances of all drives be ugly.
Samsung Support swap the 3 SSD for i can turn up again soon as possible the server. After a check, Samsung find one drive be defect and causing writes hangs.
Now, all be good. A fast raid 5 array of 3 500GB SSD for my virtual environment.
