VMware have released a KB article (213684) informing users of a bug related to Change Block Tracking (CBT) on ESXi 6.0.x. VMware have advised that the bug could cause incorrect changed blocks to be returned which may result in inconsistent backup data and could cause data loss on restore.

VMware have released a patch (VMware ESXi 6.0 patch ESXi600-201511001) which resolves this issue and this patch should be applied to all affected hosts immediately.

You can download the patch here:  


What should you do if you have been using Altaro VM Backup to backup Virtual Machines on ESXi 6.0.x?

Altaro has performed extensive testing related to this issue and the issue could only be replicated in very rare cases when the Host machine in question is under high I/O load.

However, any backup taken on an ESXi 6.0.x host with CBT enabled could potentially be affected.

Altaro is advising that in these cases users should:

  1. Patch the affected hosts
  2. Ensure that the hosts are restarted
  3. For each Virtual Machine disable CBT from the [Advanced Settings] screen in the Altaro Management console and take a successful backup
  4. After a successful backup is taken for all Virtual Machines then CBT can be re-enabled for subsequent backups.

This will ensure that all subsequent backups with CBT enabled are done correctly.

N.B. Despite this resolution, backups taken before the patch is applied may still have issues.

It is important to note that in this particular case:

  • Running Altaro's Sandbox and Verification feature is not adequate to ensure that your backups were not affected
  • Restoring a VM backup and booting it successfully will also not guarantee that the backup was not affected
  • To increase your confidence in a VM backup it is best to perform a Sandbox Restore, boot the restored VM, and verify any database files, run scripts etc.  This is still not a guarantee that the backup was not affected

Our support team is available or any queries related to this VMware issue at support@altaro.com or through the Live Chat within our product.  They are well informed about the issue and are ready to help out in any way necessary.