External qcow2 snaphots is when you create a second qcow2 file, which refers to the original as base file. New data is always written to the second file. The base file is read-only.
AFAIK ovirt is using this model.
It is a permission problem on the NAS. I guess user "root" is not allowed to create files there.
> INFO: tar: /mnt/pve/NAS/dump/vzdump-lxc-111-2024_07_14-00_30_17.tmp: Cannot open: Permission denied
Please note that a reboot is not the same as stop/start.
And no, we do not persist the dirty bitmap on stop, because we are afraid of changes from external sources.