Ah, I just remembered why RAID5 should preferrably consist of 3, 5 or 9 drives. That's a relic from the days, HDDs used to have 512 byte sectors. With 9 drives and 4k filesystem clusters (if aligned correctly) every written cluster means a single write access to every drive.

Now that HDDs have physical 4k sectors, this stands only true for multi cluster writes (or at least 32k stripe size in case of 9 HDDs)

See? Charly made his homework.

edit: Made a typo in the brackets. Not 32k stripe size, but 32k cluster size with 4k stripe size.