When any capacity device in your cluster reaches 80 percent utilization, Virtual SAN automatically rebalances the cluster, until the utilization of 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 Virtual SAN detects hardware failures on the cluster
If Virtual SAN hosts are placed in maintenance mode with the Full data migration option
If Virtual SAN hosts are placed in maintenance mode with Ensure accessibility when objects assigned FTT=0 reside on the host.
To provide enough space for maintenance and reprotection, and to minimize automatic rebalancing events in the Virtual SAN cluster, consider keeping 30-percent capacity available at all times.
You can manually rebalance the Virtual SAN cluster by using the Ruby vSphere Console (RVC). See Manual Rebalance.