Application aware image processing for Hyper-V


Information about application aware image processing for Hyper-V


Application aware image processing for Hyper-V.


This article answers the following Questions:

  1. Can Unitrends Hyper-V agent get a consistent backup of a virtual volume within a VM?
  2. Can Unitrends Hyper-V agent get a consistent backup of an application residing with a VM?
  3. Can Hyper-V agent tell the application that is has been backed up so that it can clear its transaction logs?


Unitrends Hyper-V agent offers fully-supported, VSS application-aware, transitionally-consistent backups of any applications that have a VSS writer.  These include Exchange, SQL Server, Oracle, SharePoint, and others.  There is no need to put an agent in any VM, because applications will get properly backed up and applications will know if they have been backed up, as thus clearing their logs without any agents. 

The Unitrends Hyper-V agent usees VSS to take a snapshot of the volume(s) the application resides on via VSS and then backs up that snapshot, after that it releases the application.

By default, backs up virtual machines in an online state (“Live” or “Hot” or “Online” backups).

Hyper-V VSS Writer has the ability to coordinate with VM’s VSS to back up virtual machines without taking them offline or interrupting and application aware VSS snapshot will be performed under the following conditions:

  • The Hyper-V Version 2008+ is used and is patched to it's latest available build
  • The guests in that HV host are 2003+ and are of a version and build specifically supported by the edition of Hyper-V they are on.  (note for example HV 2012 does not support 2019 guests, and 2019 does not support 2003 guests)
  • Hyper-V Integration services have been installed in the guest and are not disabled.  
  • The guest has sufficient free space and resources to perform normal VSS operations and the VSS services in the guest are healthy
  • The Hyper-V host has sufficient space to create VM snapshots (free space not less than 10% but potentially up to the size of the VMs concurrently snapshot per Microsoft Recommendations) and sufficient IO and compute resources to process the operation.  
  • All disks mounted inside the guest exist within the confines of VHD or VHDX files managed by hyper-V (no passthru disks) and all disks are intended to be protected
  • In-guest backups using an agent are not in use
  • The VM is not encrypted
  • The VM is online
  • If using a clustered HV solution, the host that currently owns the CSV disk is also the host the guest is running on (if this is not a match significant additional resource loads may be incurred which may cause undesired results, timeouts, cluster fail over, or potentially system crashes and data loss).  Guests from other cluster nodes should not be in the same CSV disk concurrently.  Should this not be the case, Unitrends and Microsoft recommend using only in-guest agent protection for affected machines.  

If the Unitrends Hyper-V agent performs an ONLINE backup, the following conditions will be true:

  1. Exchange application database files will be in consistent state and exchange will truncate transactional logs.
  2. SQL and SharePoint application database files will be in consistent state, however in the absence of SQL Transaction log backups, the database log file will continue grow in size unchecked.  Microsoft recommends SQL databases protected via host level backups utilize the SQL Simple log model to avoid this complication.  

Warning: If any of the below conditions are NOT met, then the VSS Hyper-V writer performs backups in an OFFLINE state and the VM will be put in a saved state before a snapshot of host volumes are taken for a backup (except when the virtual machine is turned off). Failure to meet these conditions would result in production impact.  

  1.  The Virtual machine (VM) guest operating system must have its own VSS support:  Vista desktops and later, Windows 2003 Server and later.
  2.  The latest Integration Services must be installed and the backup integration service must be offered. In the properties dialog of the VM, from Hyper-V Manager, look on the Integration Services tab and ensure that “Backup (volume snapshot)” is checked.
  3.  In the guest VM, the service with name "Hyper-V Volume Shadow Copy Requestor" must be running
  4.  A VM must be in a running state.
  5.  A VM must not have any dynamic disks as viewed in Disk Manager inside the guest
  6.  A VM must not have any non-NTFS file systems other than system reserve partition.  
  7.  In a cluster configuration, the virtual machine Cluster Resource Group must be ONLINE.
  8.  A virtual machine volume must have an ample free space available for the internal shadow copy to complete
  9.   “COM+ Event System”, “Distributed Transaction Coordinator”, “Remote Procedure Call (RPC)”, and “System Event Notification” services must be running within the VM. By default, these are set to “Automatic” and/or “Automatic (Delayed Start)”. The “COM+ System Application” and “Microsoft Software Shadow Copy Provider” and “Volume Shadow Copy” services must at least be set to Manual, which is the default for these. It is acceptable, but not required, to set them to “Automatic” or “Automatic (Delayed Start).
  10.   VSS Writers within VM must be in a consistent state
  11.   VSS Hyper-V Writer on the host must be in a consistent state

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