After converting vSAN stretched cluster to standard cluster VM will not power on

Products

• VMware vCenter Server

Issue/Introduction

One or more virtual machines will not power on in a vSAN cluster that was recently converted from a stretched cluster to a standard vSAN cluster. 

When attempting to power on a VM you may see the following error: Failed - Module 'MonitorLoop' power on failed.

Environment

After following the steps from the document below and successfully converting a vSAN stretched cluster into a standard vSAN cluster you may have one or more VMs that will not power on. 

Convert a Stretched Cluster to a Standard vSAN Cluster 

Prerequisites

• Back up all running VMs, and verify that all VMs are compliant with their current storage policy.
• Ensure that no health issues exist, and that all resync activities are complete.
• Change the associated storage policy to move all VM objects to one site. Use the Data locality rule to restrict virtual machine objects to the selected site.

Procedure

1. Navigate to the vSAN stretched cluster.
2. Click the Configure tab.
3. Under vSAN, click Fault Domains.
4. Disable the stretched cluster.
   1. Click Disable. The Remove Witness Host dialog opens.
   2. Click Remove to confirm.
5. Remove the fault domain configuration.
   1. Select a fault domain, and choose menu Actions > Delete. Click Yes to confirm.
   2. Select the other fault domain, and choose menu Actions > Delete. Click Yes to confirm.
6. Remove the witness host from inventory.
7. Repair the objects in the cluster.
   1. Click the Monitor tab.
   2. Under vSAN, click Health and click vSAN object health.
   3. Click Repair object immediately.
   vSAN recreates the witness components within the cluster.

Cause

It is likely the Site disaster tolerance for the vSAN storage policy has one of the following settings applied. However, these settings only apply to vSAN stretched clusters, and may prevent VMs from powering on in a standard vSAN cluster. 

• Host mirroring - 2 node cluster defines the number of additional failures that an object can tolerate after the number of failures defined by FTT is reached. vSAN performs object mirroring at the disk group level. Each data host must have at least three disk groups or three disks in a storage pool to use this rule.
• Site mirroring - stretched cluster defines the number of additional host failures that an object can tolerate after the number of failures defined by FTT is reached.
• None - keep data on Preferred (stretched cluster). If you do not want the objects in a vSAN stretched cluster to have site failure tolerance and you want to make the objects accessible only on the site that is configured as Preferred, use this option.
• None - keep data on Secondary (stretched cluster). If you do not want the objects in a vSAN stretched cluster to have site failure tolerance and you want to make the objects accessible only on the secondary site, use this option. These objects are not affected by the Inter-Switch Link (ISL) or witness host failures. They remain accessible if the site chosen by the policy is accessible.
• None - stretched cluster. If you select this option, vSAN does not guarantee that the objects will be accessible if one of the sites fails, and such objects can consume too much ISL bandwidth and can increase latency for objects that use the site mirroring policy. Use this policy only when you cannot use the other policies during some temporary condition where there is a capacity constraint (CPU/memory/storage) in the cluster.

For more information please see the following, VMware vSphere - Using vSAN Policies

Resolution

When using a standard vSAN storage policy the Site disaster tolerance setting should be set to None - standard cluster.