I would share my experience.
Scenario:
Server 1: OVH, proxmox 1.9 with 2 CT, 1 IP + 2 IPFailover, shorewall with NAT and Firewall, no mailserver
Server 2: OVH, proxmox 3.3 , 1 IP, no shorewall, default proxmox networking model, proxmox firewall inactive at moment, no mailserver
Shared NAS...