Proxmox Backup: NFS Snapshot Storage ENOSPC Impact on Guest I/O

[Alex Hung]

With VBR 13.1.1.18 and the Veeam Plug-in for Proxmox VE, when backing up a VM whose disks reside on non-snapshot-capable LVM storage and an NFS repository is configured as the Snapshot Storage, what exactly happens if the NFS Snapshot Storage runs out of space (ENOSPC) after the snapshot has already been created and the backup is in progress?

Specifically, for the QEMU copy-on-write block graph created by Veeam, which `on-cbw-error` policy is used?

* `break-guest-write`
* `break-snapshot`

If ENOSPC occurs on the NFS Snapshot Storage during an active backup, can this affect the production VM's I/O — for example, causing guest write failures, I/O errors, or the VM to be paused due to an I/O error?

We would like to confirm whether an NFS Snapshot Storage capacity exhaustion event can impact only the backup session, or whether it can also impact the running production VM.