ESXi 6.x CBT issue

VMware has released a KB article regarding a new issue affecting ESXi 6.0 and 6.0 Update 1. This issue affects backups utilizing change block tracking (CBT).

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.

VMware has identified the root cause of this issue and will release a patch to fix this issue. A release date of this patch is not available. Until then there are some “workarounds”.

  • Downgrade the affected ESXi hosts to version 5.5, and downgrade the virtual Hardware Version from 11 to 10, if necessary.
  • Shutdown the virtual machine before doing an incremental backup.
  • Do a full virtual machine backup rather than an incremental backup

Update: VMware have released a patch to fix this issue,




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