VMware vSAN 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. vSAN is implemented directly in the ESXi hypervisor.
You can configure vSAN 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 vSAN on your existing host clusters and when you create new clusters. vSAN aggregates all local capacity devices into a single datastore shared by all hosts in the vSAN cluster. You can expand the datastore by adding capacity devices or hosts with capacity devices to the cluster. vSAN works best when all ESXi hosts in the cluster share similar or identical configurations across all cluster members, including similar or identical storage configurations. This consistent configuration balances 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 vSAN datastore.
If a host contributes its local storage devices to the vSAN datastore, it must provide at least one device for flash cache and at least one device for capacity. Capacity devices are also called data disks.
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 vSAN cluster, see the VMware vSAN Design and Sizing Guide.