PowerProtect: Data Manager TSDM backup would have periodical slowness with large CBT for Windows VM

1. There is nothing wrong about PPDM/TSDM.
2. For those long-running jobs, vCenter is actually returning many changed blocks/bitmap to TSDM which makes our synthetic full TSDM backup transferred almost the full backup data even though there is little change on the VM.

If you grep "Snapshot bitmap cardinality" from vmware.log, you could see it is VMware that sends large bitmap cardinality periodically.

This is related to https://knowledge.broadcom.com/external/article?legacyId=2150251 . Certain operations on Windows VM trigger unmap commands to be sent. This includes quick formatting, when disk optimization is enabled on the drives and when files are deleted so on.

Workaround
Disable UNMAP on the affected VMs following VMware KB https://knowledge.broadcom.com/external/article?legacyId=2150251

To disable Unmap in the Windows Guest OS:
Using a Windows CMD window on the Host, run the command:

fsutil behavior set DisableDeleteNotify 1

To verify the current setting, use the following command:

fsutil behavior query DisableDeleteNotify

Permanent fix
VMware is still investigating this UNMAP issue. Contact VMware support if the above workaround is not acceptable.