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How does VMware CBT affect VMs?

How does VMware Changed Block Tracking affect virtual machines and data protection?

Part of virtual machine management is the use of data protection, including snapshots. Incremental snapshots are...

faster and more space-efficient because they are able to capture only the content that has changed since the last iteration. This allows for more frequent snapshot iterations while using considerably less storage space. It also requires less network bandwidth to replicate snapshots to remote locations for disaster recovery purposes. However, incremental snapshots usually require a mechanism to track these changes. VMware calls this mechanism Changed Block Tracking (CBT), which can be accessed by backup software tools through the vStorage APIs for Data Protection. Without VMware CBT, changed blocks would need to be tracked as a separate function through the actual backup or snapshot management tool.

If you plan to employ incremental snapshots in a VMware environment, it’s important to enable VMware CBT in the virtual machine being protected. This typically involves powering off the VM and adding a ctkEnabled parameter to the VM’s Configuration Parameters dialog according to VMware’s documentation. You can verify that VMware CBT is enabled by checking for the presence of .vmdk files with .ctk tags in each virtual disk and snapshot, such as vmname-ctk.vmdk or vmname-000139-ctk.vmdk.

Once VMware CBT is enabled, there is typically little reason to disable the feature. However, CBT can usually be disabled by setting the ctkEnabled parameter to false and removing any existing snapshots on the VM.

Next Steps

Troubleshooting options for VMware snapshots

Evaluating virtualization backup software options

VSphere Data Protection and capacity problems

Discover the true value of VMware’s CBT

This was last published in August 2015

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