Microsoft has quietly closed a long-standing operational gap for datacenter teams: Windows Server failover clusters can now host Storage Spaces Direct (S2D)-backed Cluster Shared Volumes (CSVs) and traditional SAN-LUN CSVs simultaneously within the same cluster. This previously undocumented capability, which has been available since Windows Server 2022, represents a significant evolution in Microsoft's hybrid storage strategy, allowing organizations to blend hyper-converged and traditional storage architectures without maintaining separate clusters. The mixed topology support enables more flexible storage migration paths, cost optimization through phased deployments, and better utilization of existing SAN investments while adopting modern software-defined storage technologies.

The Evolution of Windows Server Storage Architectures

Windows Server failover clustering has supported multiple storage architectures over the years, but until recently, these were largely mutually exclusive within a single cluster. Traditional SAN-based clusters utilized Fibre Channel or iSCSI LUNs presented to cluster nodes, with CSVs providing the shared storage abstraction layer. With the introduction of Storage Spaces Direct in Windows Server 2016, Microsoft offered a completely different architecture—a hyper-converged solution using direct-attached storage across cluster nodes without traditional shared storage hardware. These two approaches represented fundamentally different paradigms, and Microsoft's documentation historically indicated they couldn't coexist in the same cluster.

According to Microsoft's official documentation, Storage Spaces Direct is designed as a converged or hyper-converged solution where compute and storage resources are combined on the same servers. The technology uses local storage devices (NVMe, SSD, or HDD) across cluster nodes to create a software-defined storage pool with built-in resiliency through mirroring or erasure coding. Traditional SAN architectures, in contrast, rely on external storage arrays that present LUNs to cluster nodes over storage networks. The mixed topology capability bridges these two worlds, allowing organizations to run both architectures side-by-side.

Technical Implementation and Requirements

The mixed topology capability requires Windows Server 2022 or later, as this functionality wasn't officially supported or documented in earlier versions. When configuring a mixed topology cluster, administrators must follow specific guidelines to ensure proper operation. The cluster must first be configured with Storage Spaces Direct, which establishes the software-defined storage foundation. Once S2D is operational, traditional SAN LUNs can be added to the cluster and configured as additional CSVs. This order of operations is critical because S2D requires specific cluster configurations that differ from traditional SAN-based clusters.

From a technical perspective, the mixed topology operates through Windows Server's Cluster Shared Volume (CSV) layer, which abstracts the underlying storage technology. CSVs provide a consistent namespace and access mechanism regardless of whether the backend storage is S2D, SAN LUNs, or a combination. The CSV layer handles I/O redirection, allowing any cluster node to access storage even when it's not directly connected to the physical storage device. This abstraction enables the coexistence of different storage technologies while maintaining consistent management interfaces and failover behaviors.

Key requirements for implementing mixed topology include:

  • Windows Server 2022 Datacenter edition (required for Storage Spaces Direct)
  • Identical storage drivers and firmware across all nodes for S2D components
  • Proper SAN zoning and multipathing configuration for traditional LUNs
  • Sufficient network bandwidth for both S2D storage bus layer and SAN traffic
  • Careful capacity planning to ensure adequate performance across both storage types

Practical Applications and Migration Scenarios

The mixed topology capability addresses several real-world challenges faced by enterprise IT teams. Organizations with significant investments in SAN infrastructure can now gradually migrate to hyper-converged architectures without maintaining separate clusters during transition periods. This phased approach reduces risk and allows teams to validate S2D performance and reliability with specific workloads before committing to full migration. Additionally, mixed topology enables cost optimization by allowing different storage tiers within the same cluster—high-performance S2D using NVMe drives for demanding applications alongside cost-effective SAN storage for archival or less critical workloads.

Another practical application involves disaster recovery and business continuity planning. Organizations can configure S2D for primary production workloads while leveraging existing SAN infrastructure for backup targets or replica storage. This approach maximizes existing investments while implementing modern resilience patterns. The mixed topology also supports specialized scenarios where certain applications or services have specific storage requirements that are better served by traditional SAN capabilities, such as applications requiring specific array-based features or certifications.

Performance Considerations and Best Practices

When implementing mixed topology clusters, performance considerations become more complex than with homogeneous storage configurations. Storage Spaces Direct performance depends heavily on the storage bus layer network, typically requiring RDMA-capable network adapters (25GbE or faster) with proper switch configuration. Traditional SAN performance, meanwhile, depends on Fibre Channel or iSCSI network infrastructure and array capabilities. These different performance characteristics must be considered when placing workloads across storage types.

Best practices for mixed topology implementation include:

  • Implementing quality of service (QoS) policies to prevent storage types from impacting each other
  • Monitoring both S2D and SAN performance metrics separately and collectively
  • Placing latency-sensitive workloads on appropriate storage based on performance characteristics
  • Ensuring proper failover testing for workloads across both storage types
  • Maintaining consistent backup and recovery strategies despite storage heterogeneity
Administrators should also consider the management implications of mixed topology. While Windows Admin Center and PowerShell provide unified management interfaces, monitoring and troubleshooting may require expertise in both S2D and traditional SAN technologies. Performance baselining becomes particularly important to establish normal operating parameters for the combined environment.

Limitations and Known Issues

Despite its advantages, mixed topology implementation has certain limitations that organizations should consider. Microsoft's documentation indicates that certain advanced S2D features, such as stretch clustering for disaster recovery, may not be available in mixed topology configurations. Additionally, the initial release of this capability in Windows Server 2022 had some restrictions around CSV ownership and failover behaviors that have been addressed in subsequent updates.

Another consideration is licensing. Storage Spaces Direct requires Windows Server Datacenter edition on all cluster nodes, regardless of whether traditional SAN storage is also utilized. This represents a significant cost consideration for organizations that might otherwise use Standard edition for SAN-based clusters. Additionally, support and troubleshooting may require engagement with both Microsoft and storage array vendors when issues arise, potentially complicating resolution processes.

The mixed topology capability reflects broader industry trends toward storage flexibility and hybrid infrastructure. As organizations increasingly adopt cloud-native architectures and containerized workloads, the ability to support multiple storage backends becomes increasingly valuable. Microsoft's implementation aligns with similar capabilities in competing platforms like VMware vSAN, which also supports hybrid storage configurations.

Looking forward, this capability may evolve to support more seamless integration with Azure Stack HCI and hybrid cloud scenarios. As Microsoft continues to develop Azure Arc-enabled server capabilities, mixed topology clusters could provide more flexible pathways for cloud integration and management. The technology also positions Windows Server as a more viable platform for edge computing scenarios where storage flexibility is often required due to resource constraints and varied workload requirements.

Implementation Guidance and Migration Planning

For organizations considering mixed topology implementation, careful planning is essential. The migration path typically involves establishing a new Windows Server 2022 cluster with S2D, then migrating workloads from existing SAN-based clusters while maintaining the SAN storage as additional CSVs. This approach minimizes disruption and allows for gradual validation of S2D performance and reliability.

Key planning considerations include:

  • Comprehensive assessment of existing workloads and their storage requirements
  • Network infrastructure evaluation to support both S2D and SAN traffic
  • Skills assessment to ensure team readiness for managing hybrid storage environments
  • Development of testing and validation procedures for mixed topology scenarios
  • Creation of rollback plans in case of implementation challenges
Organizations should also consider the long-term storage strategy when implementing mixed topology. While the capability enables gradual migration, it shouldn't be viewed as a permanent state for most organizations. Clear timelines and objectives for eventual consolidation to a primary storage architecture help prevent technical debt accumulation and management complexity.

Conclusion: A Bridge Between Storage Eras

Windows Server's mixed topology capability represents a pragmatic solution for organizations navigating the transition from traditional SAN architectures to modern software-defined storage. By allowing Storage Spaces Direct and SAN-based CSVs to coexist in the same failover cluster, Microsoft has provided datacenter teams with greater flexibility in storage strategy implementation. This capability supports gradual migration paths, maximizes existing investments, and enables more nuanced storage tiering within clustered environments.

As with any hybrid technology implementation, success depends on careful planning, proper configuration, and ongoing management. Organizations should approach mixed topology as a transitional state rather than a permanent architecture, with clear objectives for eventual storage consolidation. When implemented thoughtfully, this capability can significantly reduce the risk and complexity of storage modernization initiatives while providing immediate benefits in terms of flexibility and cost optimization. The quiet introduction of this feature in Windows Server 2022 demonstrates Microsoft's continued evolution toward more flexible, enterprise-ready storage solutions that acknowledge the heterogeneous reality of modern datacenters.