Challenge
After installing the Windows May update KB4103720 (or any rollup that includes it), synthetic merges and fulls on ReFS volumes utilizing fast clone are taking significantly longer to complete.Cause
Microsoft has identified a regression in the ReFS filesystem driver included with this update that causes severe performance degradation for Veeam fast clone operations.Solution
Microsoft has identified the issue and will be releasing a fix in a future update. As a workaround, either uninstall update KB4103720 or manually rollback the ReFS driver files (refs.sys and refsv1.sys).The ReFS driver files are located under %windir%\System32\drivers and version can be checked by opening the file properties dialog and checking the Details tab.
Currently known good driver versions:
refs.sys version 10.0.14393.2097
refsv1.sys version 10.0.14393.2214
These can be extracted from KB4093120
Link to backup catalog