If you have a dual controller system do you need to split the disks into two aggrs, one per controller? - Does all I/O too an aggr go though a single controller, hence split the storage for load balancing?
Correct. Each controller is active and needs at least 1 aggregate and 1 spare per node. These are per controller and are not shared (unless a failover event). So for a cluster, you need at least 2 aggregates and 2 spares (1 per node). Spares are not shared between cluster pairs (unfortunately).
Thanks - Does each controller also have its own root vol? - Also, with a dual controller system (2040C) can I have disks in the head and disks in a single DS4243? (I have heard you may need two DS units with a dual controller array???).
The first question you can answer yourself: if the controllers need separate aggregates, of course they also have their own root volumes.
And it doesn't matter what type of disks you have, you can assign any disks to any controller. You can split the 12 internal disks in 6 for controller A and 6 for controller B (although I wouldn't recommend it). You can give each controller disks from a DS4243 shelf. Or one controller gets the internal disks and one the external ones (that's the way it is usually done)
That is correct, each controller needs 1 disk shelf.
Do you have any sources for that claim?
We have various customers that have split shelves between the two controllers. This has definitely worked with DS14 shelves before and I have not found anything that suggests that it has changed with the new SAS based shelves
As you mentioned in your previous response you can split a disk shelf and make it available to two different controllers using software based ownership. It is however not recommended as you also mentioned. I was attempting to keep my answer short... but it appears I should have elaborated a bit more.