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As a part of VMware's vision for the software-defined data center, the company released its software-defined storage feature, Virtual SAN for vSphere 5.5, in the first half of 2014.
In the book Essential Virtual SAN: Administrator's Guide to VMware VSAN, authors Cormac Hogan and Duncan Epping explore how VSAN fits into VMware's software-defined data center vision.
Hogan and Epping start Chapter 1, titled Introduction to VSAN, with a look at the software-defined data center and software-defined storage, as well as hyperconverged applications and software-only solutions.
The authors then dive into what VSAN is, which they describe as VMware's "object-based storage system and a platform for VM storage policies that aims to simplify VM storage placement decisions for vSphere administrators."
Through policy-based management, VSAN enables users to define VM-level service-level objectives. Hogan and Epping explain how VSAN is fully integrated with vSphere and supports features like VMware High Availability and vMotion. Because VSAN is built into the hypervisor, the authors say that delivers the advantage of scalability and ease of manageability and use.
From Chapter 1 of the book:
VSAN is a software-based distributed storage solution that is built directly in the hypervisor. Although not a virtual appliance like many of the other solutions out there, a VSAN can best be thought of as a kernel-based solution that is included with the hypervisor. Technically, however, this is not completely accurate because components critical for performance and responsiveness, such as the data path and clustering, are in the kernel, while other components that collectively can be considered part of the "control plane" are implemented as native user-space agents. Nevertheless, with VSAN there is no need to install anything other than the software you are already familiar with: VMware vSphere.