Backup

https://pve.proxmox.com/wiki/Backup_and_Restore

VM Backup Fleecing

On a storage that’s not thinly provisioned, e.g. LVM or ZFS without the sparse option, the
full size of the original disk needs to be reserved for the fleecing image up-front. On a
thinly provisioned storage, the fleecing image can grow to the same size as the original
image only if the guest re-writes a whole disk while the backup is busy with another disk.

Create ZFS sparse volume:

zfs create -v nvmpool/backup-fleecing -o compression=on

Add new storage using Proxmox GUI - do not forget to enable THIN provisioning!