ZFS performance degradation when populated?

Question

ZFS

Actually, everywhere they talk about the degradation of ZFS performance when overflowing, and it is recommended not to fill more than 80%.
But everywhere in the descriptions there is some kind of confusion with the terms ... I just don’t understand, the problem occurs when filling the zpool or zvol?
For example, we are going to use NexentaStor as iSCSI storage for VMware. Zpool with 12 1Tb disks - 6 mirror vdev's. The total usable capacity is 5.3Tb. Because vm's are mostly small - we create 2 zvol. And here is the main question:
it is necessary to reserve 20% for zpool (reducing the available volume to 4.2Tb), creating 2Tb in zvol. Or is it possible to create 2 zvols of 2.65Tb each and forbid filling them with more than 2Tb?

Answer 1

It's about the place on the zpool.