Getting vSAN Error: “Unexpected Number of Fault Domains”

  • Post category:VMware / VMware vSAN
  • Post last modified:January 2, 2025

The “Unexpected number of fault domains” error in VMware vSAN can be a significant issue for administrators managing a stretched cluster environment. This error indicates a configuration problem that can affect the operation of the vSAN cluster. Let’s delve into what this error means and how to resolve it.

What is the “Unexpected Number of Fault Domains” Error?

The vSAN Health Service performs various checks to ensure the cluster’s health and stability. One such check verifies that exactly two fault domains are configured for data sites in a stretched cluster. If this check fails, it triggers the “Unexpected number of fault domains” error. This error typically arises when the configuration does not meet the required criteria for fault domains, which can lead to potential data loss or unavailability.

Causes of the Error

The primary cause of this error is a misconfiguration in the fault domains of the vSAN stretched cluster. Each data site must reside in a different fault domain, and the total number of fault domains for data sites must be two. Additionally, the witness host, which acts as a tiebreaker in the cluster, should not be part of either data site’s fault domain.

Another Major cause for this issue could be Intermitent, i.e. while you are trying to add new Hosts to the cluster and they are not yet part of any Fault Domains, you would see the same error as evey node will have a fault domain of its own but its temprory and can be ignored.

Steps to Resolve the Error

To resolve the “Unexpected number of fault domains” error, follow these guidelines:

  1. Verify Cluster Configuration: Ensure that the stretched cluster has two data sites and one witness host.
  2. Configure Fault Domains Correctly: Each data site should be in a separate fault domain. The witness host should be in a different fault domain than the data sites.
  3. Use esxcli Commands: You can use the esxcli vsan faultdomain command to view and reconfigure the fault domain of each host.

Detailed Resolution Steps

  1. Check the Number of Fault Domains: Run the following command to check the current fault domain configuration:
esxcli vsan faulltdomain list

2. Reconfigure Fault Domains: If the configuration is incorrect, reconfigure the fault domains using:

esxcli vsan faultdomain add --name <FaultDomainName> --hosts <HostList>

3. Verify the Witness Host Configuration: Ensure the witness host is not part of any data site’s fault domain. If necessary, reconfigure it:

esxcli vsan faultdomain remove --name <WitnessHostFaultDomain>
esxcli vsan faultdomain add --name <NewWitnessHostFaultDomain> --hosts <WitnessHost>

Proper configuration of fault domains is crucial for the stability and performance of a vSAN stretched cluster. By following the steps outlined above, you can resolve the “Unexpected number of fault domains” error and ensure your vSAN environment operates smoothly.

Ashutosh Dixit

I am currently working as a Senior Technical Support Engineer with VMware Premier Services for Telco. Before this, I worked as a Technical Lead with Microsoft Enterprise Platform Support for Production and Premier Support. I am an expert in High-Availability, Deployments, and VMware Core technology along with Tanzu and Horizon.

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