Storage Spaces Direct (S2D)
Storage Spaces Direct (S2D) is Microsoft's software-defined storage technology in the Datacenter editions of Windows Server and in Azure Local. It pools the local drives of 2 to 16 cluster nodes through a Software Storage Bus into shared capacity from which highly available volumes are created. From Windows Server 2019 onwards a cluster supports over 400 drives and up to 4 petabytes of pool capacity; Microsoft recommends no more than roughly 400 terabytes of raw capacity per server so that resync after maintenance or reboot stays short.
Storage Spaces Direct (S2D) in practice
S2D replaces the classic SAN with direct-attached drives — SATA, SAS, NVMe or persistent memory — each owned by exactly one server. Redundancy is created in software across nodes: two-way mirror from two servers, three-way mirror from three, dual parity from four. For two-node clusters there is nested resiliency, which survives the simultaneous loss of one server and a drive in the remaining node; it requires Windows Server 2019 or later and is intended for exactly two servers only.
The minimum population under Windows Server is four capacity drives per server — two under Azure Local — plus two cache drives where a cache is used; cache devices must be at least 32 GB, and Microsoft budgets 4 GB of RAM per terabyte of cache capacity per server for S2D metadata. Nodes communicate over SMB3 with SMB Direct and SMB Multichannel; Microsoft requires at least 10 GbE for small two- to three-node clusters and recommends 25 GbE with RDMA (iWARP or RoCE) from four nodes upwards — the point where most real-world designs fail, because a 1G network throttles the cluster.
Hardware and drivers must be certified in the Windows Server Catalog for the operating system in use, and the finished cluster must pass validation via Test-Cluster. S2D is also the storage foundation of Azure Local, so a well-designed S2D cluster stays ready for a later hybrid extension.