This article is only relevant when Veeam Backup & Replication is used with the following Hyper-V versions. In these older versions of Hyper-V there was no native CBT function, and Veeam had to use it's own custom driver.
Ensure Veeam Backup & Replication is running on the latest build. Latest patches will be visible on the download page for Backup & Replication (https://www.veeam.com/download-version.html). You will need to login to the Veeam Customer Portal to obtain any downloads.
The following things will cause a warning message stating “failed to flush changed block tracking” when backing up VMs on a Hyper-V cluster:
If Hyper-V checkpoints/snapshots are manually created/deleted (merged) on VMs, you will get a message that changed block tracking is not working. This is because checkpoint/snapshot operations create an additional file (avhd/avhdx) that has to be tracked. It is added to the list of files to track (or removed in case of deletion/merge of a snapshot/checkpoint) and the changed block tracking warning should resolve the second job run after the snapshot/checkpoint was created/deleted (merged).
Should the information above not resolve the issue, changed block tracking can be reset for a Hyper-V cluster by performing the following steps:
If the issue persists, reboot each Hyper-V host in the cluster. This will typically involve migrating VMs from one host to another and rebooting the host once it does not own any VMs. Repeat these steps for all Hyper-V hosts in the cluster.