Would this still be an issue when using Backup + Stop Mode? ( Because PVE knows it is stopping the VM and will restart it shortly after)persisting the bitmap is not the issue - ensuring that it is still valid when starting the VM again is. we have no control over who/what accesses and potentially modifies the volume in-between, and any modification would lead to an invalid backup (and by the nature of incremental backups, this would propagate until the invalid chunk(s) are no longer part of the backup).
Would you be consider keeping the dirty bitmap only if used with backup in stop mode?
This would help one use case: orderly shutdown, then immediate backup.
Also, would this even be possible on storage types such as ZFS volumes or RBDs, or only with qcow2-Image Files?