You should decide on the scale and capacity of the VSA cluster and also consider some setup limitations.

Install vCenter Server on a physical host or in a virtual machine on an ESXi host. The host that runs vCenter Server can be a part of the VSA cluster.

vCenter Server must be installed and running before you create the VSA cluster.

If you run vCenter Server on a VSA datastore and the datastore goes offline, you will not be able to manage the VSA cluster due to the loss of access to vCenter Server and VSA Manager. This is not a supported configuration.

Decide on 2-member or 3-member VSA cluster. You cannot add another VSA cluster member to a running VSA cluster. For example, you cannot extend a 2-member VSA cluster with another member.

Determine the capacity of the VSA cluster before installation.

The VSA cluster requires RAID volumes created from the physical disks. The vSphere Storage Appliance uses RAID1 to maintain the VSA datastores’ replicas. See the 5.1.1 Release Notes for supported disk and RAID combinations.

Determine the number of virtual machines that will run in the VSA cluster.

Consider the vSphere HA admission control reservations when determining the number of virtual machines and the amount of resources that your cluster supports. vSphere HA admission control reserves 33% of all CPU and memory resources in a 3-member VSA cluster and 50% of all CPU and memory resources in a 2-member cluster. vSphere HA admission control makes the reservations to ensure that resources are available when virtual machines need to be restarted from a failed ESXi host onto a running ESXi host.

The VSA cluster that includes ESXi version 5.0 hosts does not support memory overcommitment for virtual machines. Because of this, you should reserve the configured memory of all non-VSA virtual machines that use VSA datastores so that you do not overcommit memory. For virtual machines that are not stored on the VSA datastores, you can skip this step. For more information about preventing memory overcommitment, see the VMware vSphere Storage Appliance Administration document.

If all cluster members run ESXi version 5.1, the VSA cluster supports memory overcommitment.