I'm reviewing my setup after some tests and I see the following:
When I zero all my disks and create a flexible root-volume random disks over all shelves are used. Because of the fact that I use a Active-Active setup where my Second Filer owns 4 disk, on the first shelf, actually the first 4 disks which becomes the root volume.
My idea is that is best, actually you have a SPOF on shelves but it's "manageble", more than over multiple shelves.
What your idea about this and are you moving root volumes to your needs ?