Bug in Changed Block Tracking (CBT) may affect your backups – take care of VMware KB 2090639

End of October 2014 VMware published a KB that describes a major bug affecting Changed Block Tracking (CBT): QueryChangedDiskAreas API returns incorrect sectors after extending virtual machine vmdk file with Changed Block Tracking (CBT) enabled

This bug affects virtual machines with CBT enabled and that virtual disk vmdk files have been expanded past any 128 GB boundary in absolute size (it is also triggered at other sizes which are a power of two from 128 GB up, eg: 256 GB, 512 GB,…).

If you use a backup software (like Veeam, IBM TSM,..) that relies on CBT and you have expanded a vmdk file past 128 GB, it is possible that your backup is corrupt or incomplete due to this bug.

At the moment there is no resolution/patch for this issue.

All you can do is the workaround described in the KB:

To work around this issue after having extended the CBT-enabled disks past a 128Gb boundary, disable and then enable Changed Block Tracking (CBT) on the virtual machine. The next backup after toggling CBT will be a full backup of the virtual machine.

Alternatively you can also trigger a Storage vMotion of the affected virtual machines – this will reset CBT, too.

1 Comment

  1. Patric

    Thank you! Didn’t hear about this yet

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