NVP vProxy: VM backup is backing up unallocated disk space as changed data

As an example, A VM has the following disk configuration and usage:

During VM backups, the following is logged in the VM backup session log:

YYYY-MM-DDT22:00:26Z INFO:   [94] Queried CBT data relative to '52 68 e2 45 c1 72 2a 28-60 2b 7c d3 20 2f b1 7c/45', for 'Hard disk 1'.
YYYY-MM-DDT22:00:27Z NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDT22:00:27Z NOTICE: [94] Used bytes = 160410107904, changed bytes = 113753849856, change rate = 070.91%.
YYYY-MM-DDT22:00:27Z INFO:   [94] File block exclusion: No objects to exclude from the backup
YYYY-MM-DDT22:00:29Z NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDT22:00:29Z INFO:   [94] Queried CBT data relative to '52 c5 cb 19 01 05 07 a7-33 ad 3c 83 ed ae 88 f3/45', for 'Hard disk 2'.
YYYY-MM-DDT22:00:29Z NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDT22:00:29Z NOTICE: [94] Used bytes = 15393120845824, changed bytes = 14654092869632, change rate = 095.20%.
YYYY-MM-DDT22:00:29Z INFO:   [94] File block exclusion: No objects to exclude from the backup
YYYY-MM-DDT22:00:29Z NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDT22:00:29Z INFO:   [94] Queried CBT data relative to '52 cd 0c c4 5d e0 d4 23-7a c3 98 f3 cc 1f 45 63/45', for 'Hard disk 3'.
YYYY-MM-DDT22:00:29Z NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDT22:00:29Z NOTICE: [94] Used bytes = 966305775616, changed bytes = 965150244864, change rate = 099.88%.
YYYY-MM-DDT22:00:29Z INFO:   [94] File block exclusion: No objects to exclude from the backup
YYYY-MM-DDT22:00:30Z NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDT22:00:30Z INFO:   [94] Queried CBT data relative to '52 eb e8 e1 8c 3d ea 1d-ff 5d 73 da 9e 91 d0 39/48', for 'Hard disk 4'.
YYYY-MM-DDTHH:MM:SSZ NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDTHH:MM:SSZ NOTICE: [94] Used bytes = 268405047296, changed bytes = 263489847296, change rate = 098.17%.
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] File block exclusion: No objects to exclude from the backup
YYYY-MM-DDTHH:MM:SSZ NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] Queried CBT data relative to '52 f7 ec 4d 6b 8e fb bb-93 5b 29 53 9a 78 6e e0/45', for 'Hard disk 5'.
YYYY-MM-DDTHH:MM:SSZ NOTICE: [94] Blocks were realigned to 0 KB boundary.
YYYY-MM-DDTHH:MM:SSZ NOTICE: [94] Used bytes = 78411005952, changed bytes = 32001884160, change rate = 040.81%.
…
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] DiskStats: Disk Label    |  Mode  | Bytes Moved |  Read Rate   |  Write Rate  |     Overhead  |   VDDK Wait   |   Total Time  | VM MORef  | VM Name                       
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] DiskStats: ------------- | ------ | ----------- | ------------ | ------------ | ------------- | ------------- | ------------- | --------- | ------------------------------
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] DiskStats: Hard disk 3   | hotadd |     899 GiB |    491 MiB/s |    179 MiB/s |       0:00:17 |        4.4 us |       1:27:52 | vm-11883  | VM_NAME                  
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] DiskStats: Hard disk 4   | hotadd |     246 GiB |    559 MiB/s |    238 MiB/s |       0:00:18 |        4.1 us |       0:21:06 | vm-11883  | VM_NAME                  
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] DiskStats: Hard disk 1   | hotadd |     106 GiB |    776 MiB/s |    222 MiB/s |       0:00:19 |        4.5 us |       0:10:29 | vm-11883  | VM_NAME                  
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] DiskStats: Hard disk 5   | hotadd |      30 GiB |    662 MiB/s |    147 MiB/s |       0:00:19 |        4.5 us |       0:04:12 | vm-11883  | VM_NAME                  
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] DiskStats: Hard disk 2   | hotadd |      14 TiB |    604 MiB/s |    252 MiB/s |       0:00:20 |        5.1 us |      16:01:26 | vm-11883  | VM_NAME                  
YYYY-MM-DDTHH:MM:SSZ INFO:   [94] DiskStats: Total         |        |      15 TiB |    596 MiB/s |    245 MiB/s |       0:01:36 |       22.7 us |      18:05:08 | vm-11883  | VM_NAME 

The VM backup session logs can be found on the vProxy under: 

/opt/emc/vproxy/runtime/logs/recycle/vbackupd/date

VM backup session logs can also be found on the NetWorker server; however, the NetWorker server logs are subject to the jobsdb retention (72 hours, default). This means that 72 hours after completion the session logs are removed from the server. The vProxy logs are not impacted by the jobsdb retention.

Linux: /nsr/logs/policy/PolicyName/WorkflowName/
Windows: ..\Program Files\EMC NetWorker\nsr\logs\policy\PolicyName\WorkflowName

Reset Change Block Tracking (CBT) for the VMs observing this issue.
See VMware documentation: Resetting Changed Block Tracking for VMware vSphere virtual machines (2139574)

It is recommended to engage VMware support if assistance with this procedure is required and to investigate any issues observed with CBT.