vSAN Skyline Health alert for "Storage Space" triggers before reaching the vCenter configured Warning threshold.
Article ID: 446891
Updated On:
Products
Issue/Introduction
Symptoms:
- A vSAN Health alert for "Storage space" triggers a warning even though the datastore utilization is below the 70% threshold visible in the UI.
- The vSAN Health "Capacity Utilization" check shows a calculated warning threshold lower than the default 70% (Ex below, Total utilization is 63.83 percent, yet the warning triggers).
- The Reservations and Alerts section also shows that current utilization has not crossed the warning threshold configured in vCenter. (For example, as shown below, hovering over the "Warning" icon displays a tooltip stating,
"Will receive a warning alert at 70%".)
- The cluster may be a stretched cluster or have specific Slack Space/Host Rebuild reservations enabled.
Environment
- VMware vSAN 7.x
- VMware vSAN 8.x (Original and ESA)
Cause
vSAN Health does not use a fixed percentage for storage limits. It dynamically calculates the threshold based on the cluster configuration and protective buffers to ensure data can be rebuilt during a failure.
The system evaluates the following formula and enforces the lowest value: Warning Threshold = min(70% of Total Capacity, Host Rebuild Threshold)
The Host Rebuild Threshold is calculated as: Total Space - Operations Reserve - Host Rebuild Reserve (Refer:vSAN Health Service - Capacity utilization - Storage space for more details about the calculations)
In a 3+3 stretched cluster configuration, these reserves can total up to 40% depending on hardware configuration (number of disks and disk groups). This results in the 'Host Rebuild Threshold' triggering a warning at 60-68% utilization. If active usage exceeds this dynamic limit, a health alert triggers even if the user-facing threshold in the vCenter UI remains set to 70%."
Resolution
To align the alert limits with your specific capacity planning requirements, configure a manual override:
Log in to the vSphere Client.
Navigate to the vSAN Cluster > Monitor > vSAN > Capacity panel.
Click the Reservations and Alerts configuration link.
Select the Customize alerts check box at the bottom of the dialog panel.
Set the custom Warning threshold (e.g., 70%) and Error threshold (e.g., 90%) to your desired values.
Click Save.
Note: Enabling Customize alerts instructs the vSAN health manager to bypass automated node-failure simulations and instead accept your static, user-defined boundaries.
Additional Information
- For long-term capacity management, review vSAN Health Service - Capacity utilization - Storage space
- To speak with a customer representative or a Support Engineer, see Contact Support.
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