Symptoms:
The nature of a distributed storage system means that data will be spread across participating nodes. vSAN manages all of this for you. Its cluster-level object manager is not only responsible for the initial placement of data, but ongoing adjustments to ensure that the data continues to adhere to the prescribed storage policy.

VMware vSAN 8.0.x
VMware vSAN 7.0.x
VMware vSAN 6.x

Data can become imbalanced for many reasons: Storage policy changes, host or disk group evacuations, adding hosts, object repairs, or overall data growth.

Note: Running a manual rebalance utilizes some system resources and this process can take several hours to complete.  This depends on the number objects that needs to be rebalanced to reduce disk usage variance across cluster.
It is recommended to run Proactive rebalance when there is minimal workload by monitoring vSAN performance charts.

There is also a health check for vSAN Disk Balance, where you can see disk usage details of the vSAN cluster. If Automatic Rebalance is enabled, vSAN automatically tries to keep this health check green. If it is disabled, this health check is triggered and will requires the admin to manually trigger a Rebalance Disks task, or re-enable Automatic Rebalance.

The toggle for enabling or disabling this cluster-level feature can be found in vCenter, under Configure > vSAN > Services > Advanced options > “Automatic Rebalance” as shown in Figure 1.


RECOMMENDATION: Keep the “Rebalancing Threshold %” entry to the default value of 30. Decreasing this value could increase the amount of resynchronization traffic and cause unnecessary rebalancing for no functional benefit.

The “vSAN Disk Balance” health check was also changed to accommodate this new capability. If vSAN detects an imbalance that meets or exceeds a threshold while automatic rebalance is disabled, it will provide the ability to enable the automatic rebalancing, as shown in Figure 2. The less-sophisticated manual rebalance operation is no longer available.



Once the Automatic Rebalance feature is enabled, the health check alarm for this balancing will no longer trigger and rebalance activity will occur automatically.
 

Should Automatic Rebalancing Be Enabled?

Yes, it is recommended to enable the automatic rebalancing feature on your vSAN clusters. When the feature was added in 6.7 U3, VMware wanted to introduce the capability slowly to customer environments and remains this way in vSAN 7. With the optimizations made to our scheduler and resynchronizations in recent editions, the feature will likely end up enabled by default at some point.

There may be a few rare cases in which one might want to temporarily disable automatic rebalancing on the cluster. Adding a large number of additional hosts to an existing cluster in a short amount of time might be one of those possibilities, as well as perhaps nested lab environments that are used for basic testing. In most cases, automatic rebalancing should be enabled.

Links
Document: https://docs.vmware.com/en/VMware-vSphere/6.5/com.vmware.vsphere.virtualsan.doc/GUID-968C05CA-FE2C-45F7-A011-51F5B53BCBF9.html
Document:  https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.vsan-monitoring.doc/GUID-968C05CA-FE2C-45F7-A011-51F5B53BCBF9.html
Release Notes: https://docs.vmware.com/en/VMware-vSphere/6.7/rn/vmware-vsan-67u3-release-notes.html

Workaround:

• Clean up space, look for VMs that are not in use or big files that can be deleted.
• Add more capacity disks to the hosts.
• Look up for VMs that are not critical or used for test and change the policy to RAID 0.
• Check if some VMs with default policy can be changed to RAID 5 or RAID 6 erasure coding.
• You can enable dedup and compression on the cluster to eliminate duplicate data.

vsan.proactive_rebalance . -h
usage: proactive_rebalance [opts] cluster
Configure proactive rebalance for vSAN
  cluster: Path to ClusterComputeResource
-s, --start            Start proactive rebalance
-t, --time-span=<i>   Determine how long this proactive rebalance lasts in seconds, only be valid when option 'start' is specified
-v, --variance-threshold=<f>    Configure the threshold, that only if disk's used_capacity/disk_capacity exceeds this threshold(comparing to the disk with the least fullness in the cluster), disk is qualified for proactive rebalance, only be valid when option 'start' is specified
  -i, --time-threshold=<i>   Threshold in seconds, that only when variance threshold continuously exceeds this threshold, corresponding disk will be involved to proactive rebalance, only be valid when option 'start' is specified
  -r, --rate-threshold=<i>    Determine how many data in MB could be moved per hour for each node, only be valid when option 'start' is specified
  -o, --stop                      Stop proactive rebalance
  -h, --help                      Show this message
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For more information about: vSAN のプロアクティブなリバランス
简体中文：vSAN 主动再平衡

Impact/Risks:
vSAN’s built-in logic is designed to take a conservative approach when it comes to rebalancing. It wants to avoid moving data unnecessarily. This would consume resources during the resynchronization process and may result in no material improvement. Like DRS in vSphere, the goal of vSAN’s rebalancing is not to strive for perfect symmetry of capacity or load across hosts, but to adjust data placement to reduce the potential of contention of resources. Accessing balanced data will result in better performance as it reduces the potential of reduced performance due to resource contention.