Problem is padding overhead when using zvols on raidz. See here to learn how to calculate usable size and how to minimize capacity loss: https://www.delphix.com/blog/delphi...or-how-i-learned-stop-worrying-and-love-raidz
Basically with a 8 disk raidz1 with default values you will loose 50% of the raw capacity and 20% of a pool should be kept free, so only 40% raw capacity is actually usable for VM disks. On paper its 3,5TB usable capacity but because everything written to a zvol will consume 175% size, the 3,5TB will be full after writing just 2TB.
Basically with a 8 disk raidz1 with default values you will loose 50% of the raw capacity and 20% of a pool should be kept free, so only 40% raw capacity is actually usable for VM disks. On paper its 3,5TB usable capacity but because everything written to a zvol will consume 175% size, the 3,5TB will be full after writing just 2TB.