So by default, Disk Pool uses RAID level 6. And for RAID level 6 you should have the tolerance of 2 drive failure before any subsequent data lose.
What the 6 Preservation Capacity Drives means is that, slices of free block are reserved on each disk in the pool that sums up to a total equal to 6 disk drives capacity. These are not any 6 physical disks that you can identify them individually.
I am glad that my explanation was accepted and gave you some clarifications. However, i am not clear if you understood 8 to be a number of physical disks that you have available. So just to further clarify
- 2 is the tolerated number of disk that can fail in the pool and there will be no concern of data lose.
- 6 is the number of free capacity that is used by storage pool for background operation which you can not take control of.
6 x 14TB = 84TB
84/180 ~= 466GB
(ie there is approx 466GB of free space reserved on each of the 180 disks)