If your vSAN cluster is running out of storage capacity or when you notice reduced performance of the cluster, you can expand the cluster for capacity and performance.
Expand the storage capacity of your cluster by adding storage devices to existing disk groups or by creating new disk groups. New disk groups require flash devices for the cache. For information about adding devices to disk groups, see Add Devices to the Disk Group. Adding capacity devices without increasing the cache might reduce your cache-to-capacity ratio to an unsupported level. See Design Considerations for Flash Caching Devices in vSAN.
Improve the cluster performance by adding at least one cache device (flash) and one capacity device (flash or magnetic disk) to an existing storage I/O controller or to a new host. Or you can add one or more hosts with disk groups to produce the same performance impact after vSAN completes a proactive rebalance in the vSAN cluster.
Although compute-only hosts can exist in a vSAN environment and consume capacity from other hosts in the cluster, add uniformly configured hosts for efficient operation. For best results, add hosts with cache and capacity devices to expand the cluster capacity. Although it is best to use the same or similar devices in your disk groups, any device listed on the vSAN HCL is supported. Try to distribute capacity evenly across hosts and disk groups. For information about adding devices to disk groups, see Add Devices to the Disk Group.
After you expand the cluster capacity, perform a manual rebalance to distribute resources evenly across the cluster. For more information, see Manual Rebalance.