This is a short summary on Storage Spaces Direct – the first true software-defined storage from Microsoft, its history and peculiarities. Software-defined storage is a concept, which involves storing data without dedicated hardware. This is an introduction to a series of posts dedicated to theoretical and practical research of S2D. The post is not a full description of the Storage Spaces Direct technology, more info is available on Microsoft resources. Our focus here is the actual problem that S2D solved for virtualization world, being truly independent from underlying hardware, as SDS should. Its predecessor, Clustered Storage Spaces, had a very serious hardware lock-in with its requirements for SAS fabrics, SAS switches and SAS JBODS. Thus, Storage Spaces Direct is an interesting technology to research.