The architecture of a VSA cluster includes the physical servers that have local hard disks, ESXi as the operating system of the physical servers, and the vSphere Storage Appliance virtual machines that run clustering services to create volumes that are exported as the VSA datastores via NFS.
vSphere Storage Appliance supports the creation of a VSA cluster with two or three members. A vSphere Storage Appliance uses the hard disks of an ESXi host to create two volumes of the same size. It exports one of the volumes as a datastore. The other volume is a replica of the volume that is exported by another vSphere Storage Appliance from another host in the VSA cluster.
VSA Cluster with Three ESXi Hosts
A VSA cluster with three members has three VSA datastores and maintains a replica of each datastore. This configuration does not require the VSA cluster service.
VSA Cluster with Two ESXi Hosts
A VSA cluster with two VSA cluster members uses an additional service called VSA cluster service. The service participates as a member in the VSA cluster, but it does not provide storage. For the VSA datastores to remain online, a VSA cluster requires that more than half of the members are also online. If one instance of a vSphere Storage Appliance fails, the VSA datastores can remain online only if the remaining VSA cluster member and the VSA cluster service are online.
A VSA cluster with two members has two VSA datastores and maintains a replica of each datastore.
In a simple configuration, the VSA cluster service can run on the vCenter Server machine.
When you use a single vCenter Server instance to manage multiple remote VSA clusters in a more complex configuration, the VSA cluster service must always run on the same network of the two member VSA cluster.
You can run the VSA cluster service on a variety of platforms in a physical or virtual machine. The installation of that VSA Cluster Service is separated from the VSA cluster installation.
However, note that the installation of VSA Manager on vCenter Server always installs the VSA Cluster Service, whether it is going to be used or not.
The following illustration shows a single vCenter Server that manages a combination of three member and two member clusters. One two member cluster uses the VSA cluster service installed on vCenter Server while the rest of the two member clusters have their own VSA cluster service.