Indeed, it wasn’t clear that it was how it worked. That seems better than the cache/mover system from Unraid.
Another point as to why I didn’t consider ZFS, is that it only works with disks having the same capacities. As I will be adding disks over time, I think I will be wasting disk space
I was comparing Proxmox and Unraid. I had ruled out TrueNAS because it only supports ZFS. I was wrong about the RAM for ZFS, but another issue is that it doesn’t support different disk sizes