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] -cbtdirfolder-disk disk.vmdk vixDiskCheck readPerf [authOptions] -cbtdirfolder-disk disk.vmdk [-disk disk2.vmdk ...] vixDiskCheck writePerf [authOptions] -cbtdirfolder-disk disk.vmdk [-disk disk2.vmdk ...]
CBT Troubleshooting
To debug CBT on a customer site, follow steps
in section Troubleshooting Changed Block Tracking (CBT)