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Backing up a Changed Block Tracking enabled virtual machine in ESXi 6.0.x returns incorrect changed sectors (2136854)

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When running virtual machine backups which utilize Changed Block Tracking (CBT) in ESXi 6.0, you experience these symptoms:
  • The CBT API call QueryDiskChangedAreas()can sometimes return incorrect changed sectors, which results in inconsistent incremental virtual machine backups.
  • Inconsistent virtual machine backups.


When running incremental virtual machine backups, backup applications typically rely on the vSphere API call QueryDiskChangedAreas() to determine the changed sectors.

This issue occurs due to a problem with CBT in the disklib area, which results in the change tracking information of I/Os that occur during snapshot consolidation to be lost. The main backup payload data is never lost and it is always written to the backend device. However, the corresponding change tracking information entries which occur during the consolidation task are missed. Subsequent QueryDiskChangedAreas() calls do not include these missed blocks and, therefore a backup based on this CBT data is inconsistent. 

All incremental backups which utilize CBT are potentially affected.


This issue is resolved in VMware ESXi 6.0 patch ESXi600-201511001. For more information, see VMware ESXi 6.0, Patch Release ESXi600-201511001 (2137545).

Required steps to ensure incremental virtual machine backup integrity

After the host upgrade is complete, you must create a full virtual machine backup to ensure incremental virtual machine backup integrity. All subsequent incremental backups must reference only the change IDs generated since the full backup (post patch install). You must ensure that the change IDs generated before the host upgrade are not referenced, as this can lead to incorrect sectors being returned by CBT. 

Important considerations:
For more information about supported virtual disk types, see Types of supported Virtual Disks on ESXi/ESX hosts (1022242).

If you are unable to upgrade, use one of these options to work around this issue:
Note: Your virtual machine backup solution vendor may have additional information about the impact of this issue on your backup solution, and specific steps that can be applied to mitigate the issue. For more information, contact your backup vendor's support.


Data loss could occur on restore from an inconsistent virtual machine backup.

See Also

Update History

11/25/2015 - Added the details of the ESXi 6.0 patch that resolves this issue. 12/03/2015 - Updated the article with important considerations to ensure consistent incremental backup integrity

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  • 183 Ratings