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VMware ESXi bug in Changed Block Tracking (CBT)

VMware ESXi bug in Changed Block Tracking

Recently VMware has published a bug on virtual disk (vmdk) file with Changed Block Tracking (CBT) enabled. The bug will render the virtual disk backup images invalid.

According to the VMware Knowledge Base Article #2090639, if the vmdk file has been extended by a size greater than 128GB, for example, 200GB to 350GB, the vmdk file will be affected by this bug. If the extension is completed 10 times, each time 20GB, the file is still affected by this CBT bug.

This issue affects all users of ESXi 4.x and ESXi 5.x. The root cause is the Changed Block Tracking information is getting corrupted when crossing these boundaries.

Recommended action

There is no official solution offered by VMware. A workaround is recommended for all users to disable and then enable CBT for all affected virtual images. All incremental backups after growing the disk should be considered invalid. Users should perform a full backup on all affected disks after re-enabling CBT.

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