Microsoft Storage Spaces Direct (S2D) is a software-defined storage solution that’s included with the Windows Server 2019 Datacenter Edition operating system. When run on industrystandard server hardware with commodity storage devices, S2D provides the shared capacity for an HCI cluster supporting virtual machines on Hyper-V, which is also included in Windows Server 2019. While this is the most common use case, S2D can also be used as a scale-out storage system for applications running on external servers.
Microsoft currently partners with over a dozen OEMs that offer Azure Stack HCI, in various hardware configurations. (See Evaluator Group Azure Stack HCI Product Brief and Hyperconverged Infrastructure Comparison Matrix).
Storage Spaces Direct runs in the Windows Server 2019 kernel, not as a VM like most other SDS solutions, providing potential performance and cost advantages. Clusters contain a minimum of two servers, a maximum of sixteen, with a max capacity of 4PB (raw). All servers must have the same number of drives and Microsoft recommends that server nodes be of the same make and model.
S2D uses Cluster Shared Volumes (CSV) as a clustered file system layer. CSV performs metadata synchronization and I/O forwarding between nodes using the SMB (Server Message Block) protocol. S2D is built on the Storage Bus Layer (SBL), a virtual bus architecture that creates a fabric connecting all disks across all nodes with using SMB as the protocol transport. With a feature called SMB Direct, SBL can use RDMA-enabled NICs (iWARP or RoCE) over 25GbE (recommended for 4+ node clusters), although SMB Direct can support smaller and larger bandwidth networks.
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