You can manually rebalance through the cluster health check, or by using RVC commands.
If the vSAN disk balance health check fails, you can initiate a manual rebalance in the vSphere Web Client. Under Cluster health, access the vSAN Disk Balance health check, and click the Rebalance Disks button.
Use the following RVC commands to manually rebalance the cluster:
vsan.check_limits. Verifies whether any capacity device in the vSAN cluster is approaching the 80 percent threshold limit.
vsan.proactive_rebalance [opts]<Path to ClusterComputeResource> --start. Manually starts the rebalance operation. When you run the command, vSAN scans the cluster for the current distribution of components, and begins to balance the distribution of components in the cluster. Use the command options to specify how long to run the rebalance operation in the cluster, and how much data to move each hour for each vSAN host. For more information about the command options for managing the rebalance operation in the vSAN cluster, see the RVC Command Reference Guide.
Because cluster rebalancing generates substantial I/O operations, it can be time-consuming and can affect the performance of virtual machines.
When you manually rebalance the disks, the operation runs for the selected time period, until no more data needs to be moved. The default time period is 24 hours. If no data is being moved, vSAN ends the rebalancing task.
You can configure an alarm that notifies you when the provisioned space reaches a certain threshold. See Creating a vCenter Server Alarm for a vSAN Event.