You disk configuration will depend greatly on how you want to handle your VM root disk. If you plan on having a limited number of nodes and/or are comfortable with your VMs root disks only living on a single system then a RAID 10, RAID5, or RAID6 setup is fine. The exact configuration would be governed by the capabilities of your RAID card and your I/O profile.

In this case the root disk for any VMs started on the R515 will only live on that host and you will need to use a block migration of some type if you want to move that VM to another host (example: nova live-migration --block-migrate <server> <host>).

If on the other hand you want your VM root disks to be able to survive the loss of a single host, or RAID controller, you need to look into some form of external storage (eg. NetApp, EMC, etc) or CEPH (http://ceph.com/) for redundant, distributed local storage.