https://pve.proxmox.com/wiki/Backup_and_Restore
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!
INFO: 21% (119.4 GiB of 564.0 GiB) in 40m 52s, read: 39.2 MiB/s, write: 0 B/s ERROR: job failed with err -13 - Permission denied INFO: aborting backup job
Fix was dded to pve-qemu-kvm=8.1.5-6
apt install pve-qemu-kvm=8.1.5-6