In addition to the online addition and subtraction of disks in VSAN, nodes can be added and subtracted online. If you wish to add more nodes to an existing VSAN cluster (a scale-out operation), the procedure is not profoundly different to the initial setup process. Once a node is added, the VSAN cluster has been expanded, and new provisioning can immediately make use of the new capacity. Storage capacity load-balancing will occur over time. vSphere DRS and HA can also make immediate use of the new node for resource scheduling and failure recovery.
Once you have acquired the new hardware and installed ESXi, you simply to need to add the host to the vCenter datacenter. Then, create a vmkernel network interface for the new node and tag it for use by VSAN (see the Configure VSAN Networking recipes in Chapter 2, Initial Configuration and Validation of Your VSAN Cluster). If necessary, tag SSDs (see the Tagging disks as local solid-state drives recipes in Chapter 2, Initial Configuration and Validation of Your VSAN Cluster or the Tagging disks as SSDs recipe in Chapter 9, VSAN 6.0). Once this is done, drag the host into the VSAN cluster and vCenter will instruct the node to join the existing VSAN cluster.
If your VSAN cluster uses auto-claim mode for disks, the disk groups will be formed automatically. If your VSAN cluster uses manual-claim mode for disks, you will need to specify which disks should be used for VSAN (see the Manually claiming disks for use by VSAN recipe in Chapter 2, Initial Configuration and Validation of Your VSAN Cluster).
When new capacity and/or nodes are added to the VSAN cluster, VSAN's placement and load-balancing logic will ultimately engage in rebalancing operations and will move components throughout the cluster according to its internal logic. This is not, however, an on-demand or user-invokable process in VSAN 5.5. It will happen automatically. In VSAN 6.0, there are ways to invoke a proactive rebalance operation using the RVC command set.