I just attempted the experiment on another Proxmox host to confirm. If I create a new zpool with the command:
zpool create POOL_NAME mirror drive1 drive 2
The new pool uses up all 70GB for a lightly used disk on a CENTOS VM. In proxmox, I have selected thin provisioning and moved the disk to...
Out of the box on Proxmox 5.x installs, the RPOOL volume is enabled with thin provisioning.
I use NFS volumes as export shares from the RPOOL. So, for example I share out /var/lib/vz which is part of the rpool. My nodes can then create VMS in that storage...