The news we’ve been waiting for is finally here: with the release a few days ago of Esxi version 8.0 U2b, VMware confirms in the release notes that the bug that afflicted the Change Block Tracking (CBT) functionality in version 8.0 U2 has been fixed.
In vSphere 8.0 Update 2, to optimize the open and close process of virtual disks during hot extension, the disk remains open during hot extend operations. Due to this change, incremental backup of virtual disks with CBT enabled might be incomplete, because the CBT in-memory bitmap does not resize, and CBT cannot record the changes to the extended disk block. As a result, when you try to restore a VM from an incremental backup of virtual disks with CBT, the VM might fail to start.
This issue is resolved in this release.
https://docs.vmware.com/en/VMware-vSphere/8.0/rn/vsphere-esxi-80u2b-release-notes/index.html
The bug, documented in official KB 95965, afflicted virtual disks with CBT enabled and hot extended, and caused possible loss or corruption of data when restoring using backup software.
https://kb.vmware.com/s/article/95965
This is because, due to the bug, some information was not being properly updated in the CBT itself, which is the mechanism used just by backup software to read only blocks that have changed since the last incremental, without having to do a full disk scan each time.
Remember, however, that the patch does not fix any VMs that had run into the bug before this update was installed.
For this reason, it is necessary to apply the workaround recommended in the above KB, meaning to reset the CBT and run a new active full.
Finally, we recommend always performing restore tests (automated or manual) to make sure that our backups are valid.