Changed Block Tracking

VMware provides the Changed Block Tracking (CBT) capability that Druva uses to analyze and backup virtual machines in an optimal way.

CBT is a VMware feature that helps perform incremental backups. CBT identifies and tracks block changes since the last backup, and stores these changes in log form. This translates to increased backup speeds, reduced backup window, and reduced size of data being transferred.

The Auto-enable CBT feature provides a way to automatically enable CBT for configured virtual machines through Druva instead of an administrator doing it manually. The CBT setting is part of backup policy. Druva will try to enable CBT before the backup schedule kicks in for virtual machine(s). There should be no user recovery points on the virtual machine for this feature to work. In case there are user recovery points present on the virtual machine, Druva will run a full scan to backup the changed blocks. To know more about CBT, see Changed Block Tracking in the VMware documentation library. 

Note:

• By default, the CBT option is enabled in the Druva backup policy.
• If CBT(Change Block Tracking) is enabled, Thin disk are restored as Thin disk.
• If CBT is disabled Thin disk are restored as Thick Disk Eager Zeroed.
• Thick Disk (Both Eager and Lazy Zeroed) are always restored as Thick Disk Eager Zeroed.
• CBT status remains unchanged if a virtual machine is restored to the original location. If a virtual machine is restored to an alternate location, CBT is disabled.  
• You may see VMWARE_VMOMI13 error if the CBT of virtual machine is not set correctly and the backup proxy version is less than 4.4. For more information on the error, see VMware VMOMI errors.
• The source data size or changed data size is determined based on the size reported by the CBT APIs of VMware.

The virtual machines list on the Management Console is updated when you deploy and register the first backup proxy on the hypervisor. This list includes the CBT status of the virtual machines hosted on the hypervisor and is refreshed every 24 hours after the backup proxy is registered. You can click the refresh button on the Management Console to manually update the CBT status of the virtual machines. 

Note: Clearing the "Auto-enable CBT" check box does not imply that CBT is disabled for virtual machine. The "Auto-enable CBT" option is only for auto enablement of CBT and not to disable CBT .

The following table illustrates the CBT state and the systems expected behavior: 

Situation	Expected Result
If CBT is disabled and " Auto‐enable CBT" : Yes
Recovery points present	Run full scan of virtual machine(s).
Recovery points not present	Enable CBT and read changed blocks for incremental backup. If for some reason Druva is not able to enable CBT, backup will fail.
If CBT is disabled and " Auto‐enable CBT" : No
Recovery points present	Run full scan of virtual machine(s).
Recovery points not present	Run full scan of virtual machine(s).
If CBT is previously enabled correctly, Auto-enable CBT --> Yes OR No
Recovery points  present	Read changed blocks for incremental backup.
Recovery points not present	Read changed blocks for incremental backup.
If CBT is enabled but in inconsistent state and  "Auto‐enable CBT": Yes
Recovery points  present	Run full scan of virtual machine(s).
Recovery points not present	CBT is reset  and read changed blocks for incremental backup.
If CBT is enabled but in inconsistent state, and "Auto‐enable CBT": No
Recovery points  present	Run full scan of virtual machine(s).
Recovery points not present	Run full scan of virtual machine(s).

Note:  Do not run the vmcontrol command and auto enable CBT run simultaneously.

To know if Druva is running a full scan, view the progress log for the job. The following is an example of the progress log for a full scan:

VMware Tools Quiescing

VMware Tools is an optional set of drivers and utilities provided by VMware to enhance the performance of the guest operating system on a virtual machine and improve interaction between the guest and the host server. The tool also provides the ability to take quiesced recovery points of Windows guest OS in conjunction with Microsoft Volume Shadow Copy Service (VSS).

Druva 'Enable VMware tools quiescing' setting provides the capability to enable or disable VMware tools quiescing if the tools are installed on the system. This functionality will ensure backups will still run by disabling the quiescing flag in case VMware tools are broken on your system. The quiesce setting is part of the backup policy. If enabled, Druva will take an application-consistent recovery point. If disabled or VMware tools not installed or running, the backup will still succeed with a crash-consistent recovery point.  You can select the Fallback to crash consistent backup option to capture a crash-consistent recovery point if quiescing of snapshot fails for reasons such as VM tools not installed/running/updated or VSS services-related issues on Guest OS and the backup fails. 

For more information, see Manage backup policy and Reconfigure virtual machines configured for backup.

Note: By default, the quiescing option is enabled and Fallback to crash consistent backup option is disabled  in the Druva backup policy.

The following table illustrates the state of  VMware tools and the systems expected behavior: 

Enable VMware tools Quiescing	Select Fallback to crash consistent	VMware tools installed	VMware tools running	VSS Service (Windows only)	Recovery point attempted
No	N/A	N/A	N/A	N/A	Crash-consistent - No Quiescing
Yes	Yes/No	No	N/A	N/A	Crash-consistent - No Quiescing
Yes	Yes/No	Yes	No	N/A	Crash-consistent - No Quiescing
Yes	No	Yes	Yes	Disabled	If "VSS service startup type" option is set to Automatic/ Automatic(Delay)/Manual,  then VMtools starts the VSS service and does quiesced recovery point. If "VSS service startup  type" option is disabled,  then recovery point operation will fail.
Yes	Yes	Yes	Yes	Disabled	Crash-consistent - No Quiescing
Yes	Yes	Yes	Yes	Enabled	Application consistent - with Quiescing This is mandatory for Microsoft SQL Server aware backups.
Yes	No	Yes	Yes	Enabled	Application consistent - with Quiescing This is mandatory for Microsoft SQL Server aware backups.