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.
What exactly is terribly slow (what benchmark do you use)?
And what kind of network do you have (1Gbit)? What kind of disks do you use for the proxmox server (model, vendor)?