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Changed Block Tracking is reset after a storage vMotion operation in vSphere 5.x (2048201)

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When running virtual machine backups using tools such as VMware Data Recovery (VDR), vSphere Data Protection (VDP), or any third-party backup tool which utilizes Changed Block Tracking (CBT) to perform incremental virtual machine backups while running on ESXi 5.x hosts, you may experience these symptoms:
  • Virtual machine backups are larger than usual
  • Incremental backups take as long and as much space as a full backup
  • Snapshot removal tasks fail because the backup job is still running or the backup window exceeds
  • CBT files grow large, although there are no major changes in the virtual machine


This issue occurs because CBT is reset during virtual disk migration using Storage vMotion. This results in the backup tool being unaware which blocks have changed since the last backup. An incremental virtual machine backup is not possible and a full backup is required.


This is a known issue affecting ESXi 5.0.

This issue is resolved in:

To work around this issue, do not use Storage vMotion or Storage DRS on virtual machines to backup.

To resolve this issue on an affected virtual machine:
  1. Shut down the virtual machine.
  2. Remove any existing virtual machine snapshots.
  3. Disable CBT for the virtual machine. For more information, see Enabling Changed Block Tracking (CBT) on virtual machines (1031873).
  4. Remove or rename any files ending with the *-ctk.vmdk file extension in the virtual machine directory.
  5. Enable CBT for the virtual machine again.
  6. Power on the virtual machine.
  7. Ensure that the virtual machine is not migrated using Storage vMotion or Storage DRS.

Additional Information

In earlier releases of VDDK, virtual machine cold migration is powered off and resulted in loss of Changed Block Tracking (CBT) state. In VDDK 5.5 release, if source and destination datastores are accessible to both hosts, CBT state is maintained after cold relocation of virtual machines.

For more information, see the Virtual Disk Development Kit (VDDK) 5.5 Release Notes.

See Also

Update History

03/11/2016 - Added the details of the ESXi 5.5 Update 2 release, which resolves this issue.

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