I am moving data from a single drive to a new volume group called "3ware" (helps me remember which controller the drives are attached to!).

The steps I took are as follows:

1. Created a PV made of /dev/md3 (two 750GB drives in RAID1).
2. Created a PV made of /dev/md9 (two 500GB drives in RAID1).
3. Added the 750GB then the 500GB physical volumes to the new volume group (in that order).

Now when I started moving data onto the new group I can see with iostat that the data is being put onto md9. So what determines which physical volumes get the data?

Good question, I'd never really thought about it before so I did a little research. Wikipedia has a decent explanation of LVM, but this website which was referenced is more definitive.

http://sunoano.name/ws/public_xhtml/lvm.html

Not a direct answer to your question, but hope I've helped.