CBT mode issue

  • 4 February 2021
  • 3 replies

Userlevel 1
Badge +2

I’m getting the error message when making a backup of a Windows VM “Failed to enable changed block tracking [Change tracking cannot be enabled when snapshots are present]”

I cannot delete this old snapshot because I need this version of the VM. Do you know any way to make backups of the VM without snapshot so I can use here “Changed Block Tracking” again?


Best answer by MichaelCapon 4 February 2021, 12:45

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3 replies

Userlevel 6
Badge +14

Hi Marius,


I believe the enabling CBT limitation is coming from VMware, It is recommended to not have any Snaps on a VM before CBT is enabled.

Ref: Enabling or disabling Changed Block Tracking (CBT) on virtual machines (1031873) (

VMware also state “Do not use a single snapshot for more than 72 hours.”


As CBT cannot be enabled due to Snapshot, The Incr job will use CRC method to calculate the changes which will take longer than CBT.


If you need the data in that Snapshot, could you create a Clone from that Snapshot?




Userlevel 7
Badge +23

Hi @Marius,

Here is how I would approach this from a VMware perspective. It involves making a new VM from the snapshot you have of the version you need to keep. Unfortunately, you cannot backup a VM without a snapshot, which would result in an inconsistent copy of the VM disks - not something you want to backup!

  1. Make a linked clone of the VM from the existing snapshot you need to preserve
    1. You need to use powercli to do it, the UI does not have the option (
  2. This will create a new VM that is linked the snapshot
  3. Clone the new linked clone VM to make it a ‘full’ independent VM (
  4. You now have a preserved version of the VM from the snapshot, but as its own VM
  5. Delete the linked-clone machine and the snapshots on the original VM
  6. Allow CV to turn CBT back on during the next backup

The downside is you will have a full copy of the VM, but chances are if you keep that snapshot around long enough it was going to consume more space than creating a copy of that VM from the older state.

The Commvault alternative is to install a guest agent and protect the machine using a file system backup instead. In the future, rather than using a snapshot on the VM as a backup of an older state, consider backing up the VM with Commvault and keeping that job around for longer!


Userlevel 1
Badge +2

Thanks guys for your feedbacks! That should fix the problem.