Files containing the name vhdx.##data##UNC are left behind in the VM''s directory.
On Hyper-V, significant storage may be used in a CSV or VM path with files containing "vhdx.##data##UNC<date>" in the name.
These files are directly related to incremental backups created by the Microsoft Hyper-v VSS writer. These files are automatically deleted following it's transfer to unitrends as part of a backup (success or failed) but may be left behind when backups are improperly interrupted due to network disconnections, VSS failures, timeouts, or unplanned reboot of hosts. This can also happen if VMs migrate between CSVs during backups.
- Please verify that there is only one backup occurring for this VM within your environment. Ensure the host node is not being backed up at the same time as guests.
- Please verify backups of VMs that live on multiple hosts but share the same CSV are not backed up in parallel
- Please review the article titled "Hyper-V 2012 Support Considerations" to ensure that you do not have any Microsoft Patches that are required from proper Microsoft VSS Writer communication.
- Run the Microsoft Best Practice Analyzer including the storage option and address any raised alerts or warnings.
- Review any VSS failures or Critical Event Log entries that do not contain "Unitrends" in the name with Microsoft support.
As long as there are no concurrent backups in flight, these ##data## files may be deleted. Please do not delete ##metadata## files are those are used in legacy hash scanning backup methods on 2008 and some newer Hyper-V hosts.
This occurs with a communication error or loss occurs during the intial phases of the backup operation, or if you have more than one Unitrends Appliance performing backups of the same VM. Primary causes include timeouts related to VSS operations on clusters that may not be deployed to microsoft's documentation standards or are using multiple VMs in the same CSV that coexist in multiple hosts (what used to be called redirected storage mode) or on SMB3 storage where IO performance may not meet production requirements.