Hi there,
On an All-SSD or AFF system, the SSDs should be split into three partitions - one root partition (R1) and two data partitions (D1, D2). This can be modified, so if the system is not fresh from the factory, more investigation is required.
The root partitions and their spares are shared between the two controllers, and in AFF configs, normally one controller will take D1 and the other will take D2. One partition is always set aside/hidden from each by the GUI - so that if an SSD fails, it can be rebuilt into the spare. From the command line, this can be overridden. Drives/partitions do not need to be marked as hot spares - in case of failure, ONTAP will use any available partition of appropriate size to rebuild onto
If all D1 and all D2 partitions are configured to one controller (active/passive), a single data aggregate of 5.9TB is created. If they are split, each controller will have one of 2.97TB, which times 2 is about 5.9TB (active/active).
With an active/passive config, less CPU cores are available, but you can run the controller to 90% CPU safely. With active/active, controller CPU should be kept under 50%, to enable failover for software updates etc. If you have a single workload, active/passive is probably better. If you have multiple workloads, active/active may be better.
Hope this helps!