When any capacity device in your cluster reaches 80 percent full, vSAN automatically rebalances the cluster, until the space available on all capacity devices is below the threshold.
Cluster rebalancing evenly distributes resources across the cluster to maintain consistent performance and availability.
Other operations can initiate cluster rebalancing:
If vSAN detects hardware failures on the cluster
If vSAN hosts are placed in maintenance mode with the Evacuate all data option
If vSAN hosts are placed in maintenance mode with Ensure data accessibility when objects assigned PFTT=0 reside on the host.
To provide enough space for maintenance and reprotection, and to minimize automatic rebalancing events in the vSAN cluster, consider keeping 30-percent capacity available at all times.
You can manually rebalance the vSAN cluster by using the Ruby vSphere Console (RVC). See Manual Rebalance.