CBT Enhancements in vSphere 7 and 8

Changed block tracking (CBT) received several important enhancements in vSphere 7.0.

For VMs of recent virtual hardware versions, CBT version 2 uses a smaller and adaptable block size. Although not user visible, this improves the resolution of tracking changes, decreases the amount of backup data, and improves perspectives for block-based recovery. As of vSphere 7.0, this feature is applied automatically when a VM is created or upgraded to hardware version 17 or higher, after CBT set or reset.

If backup customers want to have more than 1024 CBT-enabled disks open simultaneously, they can increase the CBT memory limit. CBT memory configuration is in the vSphere Client under Advanced System Settings, key MemCBTBitmapMaxAlloc. The allowed range is 128MB to 2048MB. The higher limit might allow as many open CBT disks as the ESXi open disk limit, currently 2048.

The bin64/vixDiskCheck utility was provided to verify data consistency of CBT enabled incremental backups of a VMDK. In addition to authorization options, similar to options of the VixDiskLib sample program, the -cbtdir option specifies a temporary directory that should be as large as the VMDK being checked. As of VDDK 8.0.1, the utility also measures I/O performance to help debug customer backup environments. Examples:

vixDiskCheck cbtCheck [authOptions] -cbtdir folder -disk disk.vmdk
vixDiskCheck readPerf [authOptions] -cbtdir folder -disk disk.vmdk [-disk disk2.vmdk ...]
vixDiskCheck writePerf [authOptions] -cbtdir folder -disk disk.vmdk [-disk disk2.vmdk ...]

CBT Troubleshooting

To debug CBT on a customer site, follow steps in section Troubleshooting Changed Block Tracking (CBT)