VMware Virtual SAN uses a software-defined approach that creates shared storage for virtual machines. It virtualizes the local physical storage resources of ESXi hosts and turns them into pools of storage that can be divided and assigned to virtual machines and applications according to their quality of service requirements. Virtual SAN is implemented directly in the ESXi hypervisor.
You can configure Virtual SAN to work as either a hybrid or all-flash cluster. In hybrid clusters, flash devices are used for the cache layer and magnetic disks are used for the storage capacity layer. In all-flash clusters, flash devices are used for both cache and capacity.
You can activate Virtual SAN on your existing host clusters and when you create new clusters. Virtual SAN aggregates all local capacity devices into a single datastore shared by all hosts in the Virtual SAN cluster. You can expand the datastore by adding capacity devices or hosts with capacity devices to the cluster. VMware recommends that the ESXi hosts in the cluster share similar or identical configurations across all cluster members, including similar or identical storage configurations. This ensures balanced virtual machine storage components across all devices and hosts in the cluster. Hosts without any local devices also can participate and run their virtual machines on the Virtual SAN datastore.
If a host contributes its local storage devices to the Virtual SAN datastore, it must provide at least one device for flash cache and at least one device for capacity, also called a data disk.
The devices on the contributing host form one or more disk groups. Each disk group contains one flash cache device, and one or multiple capacity devices for persistent storage. Each host can be configured to use multiple disk groups.
For best practices, capacity considerations, and general recommendations about designing and sizing a Virtual SAN cluster, see the VMware Virtual SAN Design and Sizing Guide.