Add explicit guidance for CSV file system formats (NTFS for SAN, ReFS for S2D)

Co-authored-by: robinharwood <19212983+robinharwood@users.noreply.github.com>

Author: Copilot

WindowsServerDocs/failover-clustering/clustering-requirements.md

@@ -64,10 +64,13 @@ Some clusters have a disk that functions as the *disk witness*, which is a disk
 You should also consider these additional storage requirements:
 
 - When you use native disk support included in failover clustering, you should only use basic disks, not dynamic disks.
-- We recommend you format disk partitions with New Technology File System (NTFS). If you use Cluster Shared Volumes (CSV), you must format the partitions for each CSV with NTFS.
+- We recommend you format disk partitions with New Technology File System (NTFS). If you use Cluster Shared Volumes (CSV), you must format the partitions for each CSV with NTFS or Resilient File System (ReFS) based on the following guidelines:
+
+  - **SAN volumes**: Format with NTFS before adding to CSV. NTFS enables Direct I/O mode for better performance.
+  - **Storage Spaces Direct (S2D) volumes**: Format with ReFS before adding to CSV. ReFS provides data integrity features and optimizations for Storage Spaces Direct.
 
   >[!NOTE]
-  >If you have a disk witness for your quorum configuration, you can format the disk with either NTFS or Resilient File System (ReFS).
+  >If you have a disk witness for your quorum configuration, you can format the disk with either NTFS or ReFS.
 
 - For the partition style of the disk, you can use either a Master Boot Record (MBR) or a GUID Partition Table (GPT).
 

WindowsServerDocs/failover-clustering/failover-cluster-csvs.md

@@ -51,6 +51,11 @@ Before using a CSV in a failover cluster, review the network, storage, and other
 
   In Windows Server 2012, a disk or storage space for a CSV must be a basic disk that is partitioned with NTFS. In Windows Server 2012 R2, a disk or storage space for a CSV must be a basic disk that is partitioned with NTFS or ReFS. In Windows Server 2016 and later, and Azure Local, a disk or storage space for a CSV must be either a basic disk or GUID Partition Table (GPT) disk that is partitioned with NTFS or ReFS. A best practice is to also review the guidelines that are provided by your storage vendor.
 
+  For optimal performance and functionality, use the following file system guidelines when formatting disks before adding them to CSV:
+
+  - **SAN volumes**: Format with NTFS before adding to CSV. NTFS enables Direct I/O mode for SAN-attached storage, which provides better performance. CSVs formatted with ReFS on SANs operate in redirected I/O mode.
+  - **Storage Spaces Direct (S2D) volumes**: Format with ReFS before adding to CSV. ReFS provides data integrity features, block cloning, and optimizations specifically designed for Storage Spaces Direct workloads.
+
   - In Windows Server 2012, you can't use a disk as a CSV formatted as FAT, FAT32, or ReFS.
 
   - In Windows Server 2012 R2 and later, you can't use a disk as a CSV formatted as FAT or FAT32.

WindowsServerDocs/failover-clustering/failover-cluster-manage-cluster-shared-volumes.md

@@ -21,7 +21,7 @@ To manage CSVs, ensure the following prerequisites are met:
 
 - Your cluster configuration must have at least two nodes configured in a failover cluster. All nodes must be joined to the same Active Directory (AD) domain.
 
-- At least one shared disk (LUN) must be accessible by all cluster nodes and configured as a cluster disk using NTFS or ReFS. If you're using iSCSI or Fibre Channel for shared storage, ensure that multipath I/O (MPIO) is properly configured to provide redundancy and optimal performance.
+- At least one shared disk (LUN) must be accessible by all cluster nodes and configured as a cluster disk using NTFS or ReFS. Format SAN volumes with NTFS and Storage Spaces Direct (S2D) volumes with ReFS for optimal performance. If you're using iSCSI or Fibre Channel for shared storage, ensure that multipath I/O (MPIO) is properly configured to provide redundancy and optimal performance.
 
 - At least two network interfaces per node are recommended, one for cluster communication and one for client access. Ensure that the network settings allow for proper communication between nodes, including correct IP addresses and subnet configurations. Make sure firewalls are configured to permit necessary cluster traffic.
 

WindowsServerDocs/storage/refs/refs-overview.md

@@ -142,7 +142,7 @@ Contact application and storage array vendors for support details on Fiber Chann
 | Page file support | ✅ | ✅ |
 
 1. Available on Windows Server, version 1709 and later, Windows Server 2019 (1809) LTSC or later.
-2. CSV won't use Direct I/O with Storage Spaces, Storage Spaces Direct (S2D), or SAN.
+2. ReFS-formatted CSVs won't use Direct I/O with Storage Spaces, Storage Spaces Direct (S2D), or SAN. For SAN storage, format CSVs with NTFS to use Direct I/O.
 3. Available on Windows Server 2025 only.
 4. Storage Spaces only.