production Windows Server 2025 Failover Cluster.

Hello,

What you are seeing is expected behavior in Windows Server 2025. The FileSystemRedirectedIOReason = IncompatibleFileSystemFilter flag is triggered because the Commvault CVDLP filter driver is not classified as CSVFS-compatible. Cluster Shared Volumes (CSV) operate in a highly constrained environment where only certain file system filters are supported. When an incompatible filter is attached, CSVFS forces the volume into redirected I/O mode to maintain consistency and prevent corruption.

Microsoft’s definition of IncompatibleFileSystemFilter is exactly this: any minifilter driver that does not meet CSVFS compatibility requirements will cause the cluster to redirect I/O through the coordinator node. In your case, fltmc shows CVDLP.Encryption and CVDLP.Shredding attached to the CSV, which are not recognized as CSV-aware filters. This is why both nodes report the same redirected state.

This is not a bug in Windows Server 2025 itself, but rather a compatibility gap between Commvault’s CVDLP driver set and CSVFS. Unless Commvault releases updated filter drivers certified for CSVFS on Server 2025, the cluster will continue to classify them as incompatible. The correct path forward is to confirm with Commvault support whether version 11.40.0 of their agent has CSVFS certification for Windows Server 2025. If not, you will need to either upgrade to a supported build or remove the filter from the CSV path to restore direct I/O.

To summarize: Windows Server 2025 is behaving as designed. The redirected mode is expected when non-CSV-aware filters are present. This is a compatibility issue with Commvault’s filter driver, not a defect in Server 2025.

I hope you've found something useful here. If it helps you get more insight into the issue, it's appreciated to accept the answer. Should you have more questions, feel free to leave a message. Have a nice day!

HP.