As from version 7.6, VM backup has revamped the way we perform Changed Block Tracking (CBT) for Hyper-V virtual machines running on Windows Server 2012 and 2012R2.

VM Backups's new CBT v2 for Windows Server 2012 and 2012 R2 will make backups even faster and gives VM Backup several advantages when it comes to changed block tracking:

• CBT v2 for Windows Server 2012 and 2012 R2 leverages Microsoft’s Volume Shadow Copy Service (VSS) on the hosts. VSS has been part of all versions of Microsoft Windows for over a decade, and is extremely reliable in maintaining a point-in-time snapshot of a specific volume.
  
  
• CBT v2 for Windows Server 2012 and 2012 R2 no longer requires the installation of a kernel-mode filesystem driver to track changes.
  
  
• The previous method of using filesystem drivers in VM Backup 7.5 and earlier may affect I/O performance on some systems, and this will now be completely avoided with CBT v2 for Windows Server 2012 and 2012 R2.
  
  
• With CBT v2 for Windows Server 2012 and 2012 R2, change tracking keeps working reliably across host reboots, system upgrades etc.
  
  
• CBT v2 for Windows Server 2012 and 2012 R2 also fully supports VMs migrating across cluster nodes and there is no longer need to perform a full scan when a VM migrates or changes state etc.

Notes

• Updated storage requirements: CBT v2 for Windows Server 2012 and 2012 R2 requires that one shadow copy snapshot be maintained per volume containing backed up VMs. For example if a host has VMs on C: and D:, there will be two shadow copies (one for each volume) on the system at any one time. This is the equivalent of having a System Restore Point on a server and is not expected to affect performance in any way. However due to shadow storage requirements, there may be some free space that is temporarily occupied by the system, but this is capped and flushed periodically and will never be allowed to affect a running system.
  
  

• This change affects Windows Server 2012 and 2012R2 only, and does not affect VMware and Windows Server 2016 or newer, which have a CBT API supplied by the hypervisor with which VM Backup interfaces directly.
  
  
• CBT is not supported on 2012 / 2012r2 hosts where the VHDX files are stored on an SMB3 share (Network Paths and SOFS)