vSAN Health Service - Cluster health

Products

VMware vSAN

Issue/Introduction

This article explains the Cluster Health - vSAN Disk Balance check in the vSAN Health Service and provides details on why it might report an error.

Environment

VMware vSAN (All Versions)

Resolution

Q: What does the Cluster Health - vSAN Disk Balance check do?

Every disk in an ESXi host has a total capacity, and also the percentage of used space. Currently, we use (actual physical disk usage)/(physical disk capacity) as the disk usage.
vSAN system attempts to balance the load balance between disks in the vSAN cluster, but at any given moment the load per disk varies.
The vSAN disk balance test checks each healthy disk in the vSAN cluster, and gets a summary of the average disk usage, maximum disk usage, average load variance, and maximum load variance.
If a disk's load variance exceeds the threshold, that disk would be considered unbalanced in relation to the rest of the vSAN capacity disks. In this case, another tab in the alarm shows the detailed information for these disks, including the percentage exceeding, the threshold, and data that should be moved to bring the variance below the threshold.

Q: What does it mean when it is in an error state?

If this check is yellow, it means that the disk load variance between some capacity tier disks within the cluster exceeded the threshold at some point in time.
The default threshold is set to 30%. So if disk A's usage is 45% and disk B's usage is 10%, the load variance between disk A and disk B is 35%, which exceeds the threshold.

Q: How does one troubleshoot and fix the error state?

Automatic rebalance is provided, however is not enabled by default, the user just needs to click the button 'Configure Automatic Rebalance', and it will redirect to the UI for automatic rebalance configuration. Once it's enabled, vSAN will take care of balancing the cluster.

Steps to enable Automatic Rebalance.

1. Log in to the vSphere Client.

2. Navigate to the vSAN Cluster.

3. Select the Configure tab.

4. Under vSAN, select Services.

5. Click Edit on Advanced Options.

6. Toggle Automatic Rebalance to Enabled and verify the threshold is set to the default value of 30%. If needed this value can be changed based on the requirement.

7. Click Apply to save the configurations.

Once object relocation normalizes disk utilization variance below the configured threshold, the vSAN Skyline Health alert automatically clears.

ESXi 7.x

ESXi 8.x

If vSAN Disk Balance is in a warning state check the following:

automatic rebalance is enabled, if it's not enable it
all disks are on the same on-disk format version, if not upgrade the on-disk format so all disks are on the same version

If automatic rebalance is enabled and all disks are on the same on-disk format version and this warning is triggered then disks are just running out of space and Disk capacity will also be triggered showing disks low on space.