The Storage Spaces Direct stack includes the following, starting from the bottom:
Hardware: The storage system consisting of a minimum of four storage nodes with local storage. Each storage node can have internal disks, or disks in an external SAS connected JBOD enclosure. The disk devices can be SATA disks, NVMe disks or SAS disks.
Software Storage Bus: The Software Storage Bus spans all the storage nodes and brings together the local storage in each node, so all disks are visible to the Storage Spaces layer above.
Storage Pool: The storage pool spans all local storage across all the nodes.
Storage Spaces: Storage Spaces (aka virtual disks) provide resiliency to disk or node failures as data copies are stored on different storage nodes.
Resilient File System (ReFS) ReFS provides the file system in which the Hyper-V VM files are stored. ReFS is a premier file system for virtualized deployments and includes optimizations for Storage Spaces such as error detection and automatic correction. In addition, ReFS provides accelerations for VHD(X) operations such as fixed VHD(X) creation, dynamic VHD(X) growth, and VHD(X) merge.
Clustered Shared Volumes: CSVFS layers above ReFS to bring all the mounted volumes into a single namespace.
Scale-Out File Server This is the top layer of the storage stack that provides remote access to the storage system using the SMB3 access protocol.
Windows Server Technical Preview introduces Storage Spaces Direct, which enables building highly available (HA) storage systems with local storage. This is a significant step forward in Microsoft Windows Server software-defined storage (SDS) as it simplifies the deployment and management of SDS systems and also unlocks use of new classes of disk devices, such as SATA and NVMe disk devices, that were previously not possible with clustered Storage Spaces with shared disks.