Backing up Virtual Machines in ESXi 6.0.x with Changed Block Tracking enabled returns incorrect changed sectors


VMware has announced a bug related to ESXi 6.0.x whereby backing up a changed block tracking enabled virtual machine in ESXi 6.0.x returns incorrect changed sectors.


The Issue:
Virtual machine backups utilizing Changed Block Tracking (CBT) in ESXi 6.0 may experience the following symptoms:

  • The CBT API call QueryDiskChangedAreas() API call can sometimes return incorrect changed sectors, which results in inconsistent incremental virtual machine backups.
  • Inconsistent virtual machine backups

This affects all backup vendors backing up ESXi 6.0.x via snapshots or incremental backups.  End users may experience corrupted or inconsistent backups, causing data loss on restore. VMware has announced a bug related to ESXi 6.0.x: Backing up a Changed Block Tracking enabled virtual machine in ESXi 6.0.x returns incorrect changed sectors (2136854).

This issue has been resolved in VMware ESXi 6.0 patch ESXi600-201511001. See VMware ESXi 6.0, Patch Release ESXi600-201511001 (2137545) for further details. Questions relating to this patch should be directed to VMware Support (see How to file a Support Request in My VMware).
After applying the patch, a host reboot is required. In addition, you must reset CBT on your virtual machines (see: How to Reset Change Block Tracking (CBT) for VMware Backups) and run a full backup to ensure data is consistent going forward.

Optional Work Arounds:
In the event upgrading to the patch above is not possible in the near future, VMware has suggested using one of the following workarounds.
Important Considerations:
Additional Resources:
Unitrends has created a script which may be used to reset CBT, enable/disable CBT, and get current status across multiple VMs.  See VMware CBT Reset PowerShell Cmdlet for instructions and to download the script.

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