In certain cases, CBT is skipped and the virtual machines will be scanned for changes. The below list outlines situations where CBT tracking data is skipped:


1. Taking a first backup

When a VM is backed up for the first time Altaro will go through the entire VM as there will be no CBT data available.

2. 48 Hours elapsed since the previous backup

CBT v2 for Windows Server 2012 and 2012 R2 is based on persistent ShadowCopies upon which a BITMAP file is created containing the changes between ShadowCopies. These BITMAP files are only kept for 48Hrs. If a VM is not backed up within the timeframe then the next backup will skip CBT and scan for changes during the backup operation. CBT will continue from that operations onwards unless another 48Hrs are elapsed between backups.

3. ShadowCopies are manually removed from the host volumes

In the event that a ShadowCopy created on a host or CSV is cleared manually by the administrator or a thirds party application, CBT will be skipped for the next backup. Once the new ShadowCopy is taken upon the next backup operation, CBT will then be available for subsequent backups.

4. Sparse Virtual Hard Disks (VHDX) files are not supported