Hyper-V Changed Block Tracking Troubleshooting

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.

• All nodes in the cluster must:
  • not be in the maintenance mode
  • have the Cluster Service started
  • be powered on
  • be accessible by the Veeam server
    
• Ensure all nodes within the cluster can communicate with each other.
  
  This can be tested by pinging between the nodes and using wbemtest (see http://www.veeam.com/kb1926) to connect between nodes via WMI. For wbemtest, the two namespaces you want to ensure can be connected to are root\cimv2 and root\virtualization (root\virtualization\v2 for Windows Server 2012+).
  
• If a Failover Cluster exists in the environment it must be added to the Managed Servers section in Veeam Backup & Replication. Adding the individual nodes of that cluster as standalone hosts will prevent changed block tracking data from being replicated to all nodes of the cluster.

• Ensure the SMB server is added to Veeam Backup & Replication.
  

  
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• Ensure the shares are visible in the Backup & Replication console.

  
  
  
  

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:

1. Right-click on each host in the cluster, select Manage Volumes and then tick the box to "Disable changed block tracking for this host (not recommended)"
   

   
   
   
   
    

2. In an elevated command prompt, on each Hyper-V host in the cluster, Run sc stop VeeamFCT,  to ensure the CBT service is stopped.
3. On each of the Hyper-V hosts in the cluster, go to, export, and then delete the following registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\VeeamFCT\Parameters\CTDB
4. Go back to the Manage Volumes for each host and uncheck the disable changed block tracking option. 
5. Test the job again. The job may have to be run at least twice to ensure CBT is working.

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.