Virtual SAN virtualizes local physical storage resources of ESXi hosts and turns them into pools of storage that can be carved up and assigned to virtual machines and applications according to their quality of service requirements.
You can activate Virtual SAN when you create host clusters or enable Virtual SAN on existing clusters. When enabled, Virtual SAN aggregates all local storage disks available on the hosts into a single datastore shared by all hosts. You can later expand the datastore by adding storage devices or hosts to the cluster.
The hosts in the Virtual SAN cluster do not need to be identical. Even the hosts that have no local disks can participate and run their virtual machines on the Virtual SAN datastore, as, for example, host 2 in the illustration.
If a host contributes its local storage to the Virtual SAN datastore, the host must provide one SSD and at least one HDD, also called data disk. The disks on the contributing host form a disk group. Each disk group must include one SSD and at least one or multiple data disks. The disk group uses the SSD disk for read caching and write buffering, while the data disks are used for persistent storage. You can have multiple disk groups per host. For example, in the illustration, host 1 contains one disk group, while host 3 has two disk groups.
Virtual SAN and Virtual Machine Storage Policies
When enabled, Virtual SAN works together with virtual machine storage policies. You use storage policies to define virtual machine storage requirements such as performance and availability in the form of a profile. The policy requirements are pushed to the Virtual SAN layer when the virtual disk is created. Virtual SAN lays out the virtual disk across the Virtual SAN datastore to meet the specified requirements.
Virtual SAN monitors and reports on the policy compliance during the lifecycle of the virtual machine. If the policy becomes noncompliant because of a host, disk, or network failure, Virtual SAN takes remedial actions. It reconfigures the data of the affected virtual machines and optimizes the use of resources across the cluster. Any reconfiguration processes occur with minimal impact on the regular workload.
For information on how Virtual SAN and virtual machine storage policies interact, see Virtual SAN and Storage Policy-Based Management.
For general information about storage policies, see About Virtual Machine Storage Policies.