vSAN Health Service - Cluster health - vSAN Disk Balance

Products

• VMware vSAN

Issue/Introduction

One or more of the following Symptoms are observed:

 

• vSphere Client --> Cluster events showing “vSAN Health Test 'vSAN disk balance' status changed from 'green' to 'yellow'

 

• In the vSAN Healthcheck the Warning "VSAN DISK BALANCE" is observed:
  
  
  

Example:

 

 

 

 

Environment

VMware vSAN

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 42% and disk B's usage is 10%, the load variance between disk A and disk B is 32%, which exceeds the threshold.

(as shown in the Screenshot above)

 

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

Once it's enabled, vSAN will take care of balancing the cluster.

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

 

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

• Automatic rebalance is enabled, if it's not enable it

  It is not enabled by default, the user just needs to click the button 'Configure Automatic Rebalance' (as shown in screenshot above) 

  and it will redirect to the UI for automatic rebalance configuration.

  For more information see "Configure Automatic Rebalance in vSAN Cluster"

• 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, all disks are on the same on-disk format version and this Warning is triggered:

Disks are just running out of space and Disk capacity will also be triggered showing disks low on space.

 

Additional Information

Rebalance:

• vSAN "Proactive rebalance" and "Automatic Rebalance
• Tech Docs : Configure Automatic Rebalance in vSAN Cluster
• Should Automatic Rebalancing be Enabled in a vSAN Cluster?

 

Disk Format:

• vSAN Health Service - Cluster Health - Disk format version
• Understanding vSAN on-disk format versions and compatibility