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 either by adding storage devices to existing disk groups or by creating a new disk group. 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 to provide smooth operation.
For best results, add hosts configured with both cache and capacity devices. For information about adding devices to disk groups, see Add Devices to the Disk Group.