Error Message

The byte count of a Full, Differential or Incremental backup job represents the size of data read from the source VM and written to the backup storage device.
As part of this backup process a dynamic virtual hard disk is created in the backup set folder using a certain block size.
In some situations, as example when a smaller amount of data is written to such a block, some unused spaces is being created which results in the dynamic virtual hard disk to be larger than the actual data.
Another possible reason for a larger dynamic virtual hard disk could be disk fragmentation.

A duplicate job copies each file from the backup storage device to the duplicate target device.
Therefore if the actual size within the backup target device is different because of the above explained reasons, a difference in byte count between backup and duplicate job is visible in the Backup Exec GUI.

This issue is resolved in Backup Exec 21.3

Additional steps required to reduce the difference in byte count once Backup Exec 21.3 is installed:

Warning: Incorrect use of the Windows registry editor may prevent the operating system from functioning properly. Great care should be taken when making changes to a Windows registry. Registry modifications should only be carried-out by persons experienced in the use of the registry editor application. It is recommended that a complete backup of the registry and workstation be made prior to making any registry changes.

Browse to the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Veritas\Backup Exec For Windows\Backup Exec\Engine\HyperV Agent

Create the following two registry keys as DWORD
- Virtual disk block size written in MB for full
- Virtual disk block size written in MB for diff and incr

The default value is:
- 32 for "Virtual disk block size written in MB for full"
- 2 for "Virtual disk block size written in MB for diff and incr"

By reducing the block size, a possible performance decrease or increased CPU usage might be experienced.
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To reduce the difference in byte count, the following values can be tested:
- 8 or 4 or 2 for for "Virtual disk block size written in MB for full"
- 1 for "Virtual disk block size written in MB for diff and incr"

There is a possibility that even after the block size has changed, the byte count is not identical.