How to uninstall and re-install CBT on Hyper-V


Resetting Hyper-V CBT when incrementals result in the full backup size


If Hyper-V CBT was never installed correctly it may be necessary to uninstall and then re-install the driver.  This may occur when updating to a new agent release.  In some other cases, Unitrends Support may recommend removal and re-installation of the CBT agent as well. 


Please keep in mind that the below process or installing the Unitrends agent via the MSI on a Hyper-V Host whether clustered or not can cause an unexpected reboot. Please perform these operations when migrations are possible or when a small amount of downtime is acceptable.

On agent version 9.0 or later:

1. Stop all scheduled backups for the host or cluster for the duration of this process. (Make sure you are not accessing any files or folders in "C:\unicbt". This includes using windows explorer or a command window.)

2. On each node of the cluster, uninstall the CBT driver (it should be removed from every node before it is reinstalled on any node). 

cd C:\PCBP\Win2012.dir\12-04-2015,9.0.0-6

(10-20-2015, is the prior version, if you do not find the 12-04-2015,9.0.0-6 folder, but do find the folder, then this confirms the prior update did not succeed.  If you find yet older folders perform the uninstall from that release.)

3.  Run the following to verify that the CBT driver has been uninstalled.  

PS C:\Windows\system32> fltmc

Filter Name                     Num Instances    Altitude    Frame
------------------------------  -------------  ------------  -----
DfsDriver                               0       405000         0
CsvNSFlt                                1       404900         0
CsvFlt                                  2       404800         0
DpmFilter                               4       385300         0
phdcbtdrv                               4       368340         0
msnfsflt                                0       364000         0
CCFFilter                               1       261160         0
DfsrRo                                  0       261100         0
ResumeKeyFilter                         2       202000         0
svhdxflt                                2       135100         0
luafv                                   1       135000         0
npsvctrig                               1        46000         0
​*If you see phdcbtdrv in the output, the CBT driver was not removed correctly.  This may occur if the dll is in use when the uninstall process was attempted.  To ensure the dll is not in-use, reboot the server and repeat the uninstallation process above.

If you do not see phdcbtdrv above and the C:\unicbt directory still exists, you can safely remove it before reinstalling the CBT driver.

4. When all drivers have been removed, assuming the unitrends agent release is already the current release, reinstall the CBT driver for each node.  

Note: if the Unitrends agent release is not current, pause here, uninstall the old agent, then install the new agent.  Installation of the new agent may install the CBT driver properly for you.  If not, proceed below:  
It may be necessary to reboot the host if the reinstall has prior failed.  For a cluster, migrate all guests and storage to another node and then reboot, ensure all nodes in a cluster run the current release of the CBT driver before re-enabling backup schedules, this may need to be repeated for each node, then reboot the node.  Proceed with the step below:

cd C:\PCBP\Win2012.dir\12-04-2015,9.0.0-6
.\Install.bat /i

5. On each node, verify that the driver is installed and running.

cd C:\PCBP\Win2012.dir
Results are logged to C:\PCBP\Win2012.dir\IsCBTDriverInstalled.log

6. If your CBT agent versions is, follow the KB titled Hyper-V CBT driver memory leaks to resolve additional issues to ensure the driver will function as expected.  Note, if you rebooted the host per steps above, an additional reboot, recommended in the deployment of this patch, is not necessary.
7. Do a full backup and then an incremental backup of each guest to verify that the problem is resolved. 
8. Re-enable backup schedules.
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