Changed Block Tracking (CBT) is a feature that has been around for a while within VMware virtual machines. CBT is available if you’re running ESX 4.0 or higher with hardware version 7 or higher on the VM. Basically, VMware can keep track of disk sectors that have changed since the last backup. When that VM is backed up again, and CBT is enabled, the backup will only need to copy the changed blocks.
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