Conditions that determine online vs. offline backup for VSS
This topic applies to the NetBackup Hyper-V VSS backup method only (not to WMI).
With the VSS backup method, a Hyper-V online backup can be performed with no downtime on a running virtual machine when all of the following conditions are met:
The Hyper-V integration services are installed and the VSS integration service is enabled.
All virtual machine disks are configured within the guest operating system as NTFS-formatted basic disks. If the virtual machine uses dynamic disks or the FAT or FAT32 file system, an online backup cannot be performed.
Note that "dynamic" disks are not a type of virtual hard disk (vhd or vhdx format). Dynamic disks and basic disks are disk types defined by Microsoft for certain Windows operating systems.
Each virtual machine volume must specify itself as the storage location for its shadow copies. For example, the shadow copy storage for C:\ must be configured on the C:\ volume. The shadow copy storage for D:\ must be on D:\, and so forth.
If these conditions are not met, the backup is performed offline. An offline backup results in some downtime on the virtual machine. If the virtual machine is in the Running or Paused state, it is put in a Saved state as part of the offline backup. After the backup is completed, the virtual machine is returned to its original state.