I have currently 17 disks of 2.4TB with 1 spare (RAID 6) and 2 pools. I still have 7 empty slots. If I were to populate the 7 empty slots with disks of the same size; using the same RAID configuration,  how much additional usable capacity I will get? Can I merge the new additional expansion to the current pool without breaking them as there are some data in it?