vSAN Health Alarm: Thick-provisioned VMs on vSAN (VMware vSAN)

Products

• VMware vSAN

Issue/Introduction

This article provides guidance for resolving the Skyline Health alarm "Thick-provisioned VMs on vSAN." This alert identifies virtual machine disks on the vSAN datastore that are thick-provisioned, despite policies typically defaulting to thin provisioning. This mismatch often occurs following VM migrations or manual policy modifications.

Symptoms

• The alarm may persist for an already deleted VM due to cached metadata.

• The overview section within the alert will detail the disks in question.

 

Environment

VMware vSAN 8.x

Cause

This health alarm triggers when vSAN detects virtual machine objects with Object Space Reservation (OSR) set to 100%, indicating thick provisioning, while the active VM Storage Policy expects 0% (Thin). Common causes include:

• Migration Artifacts: Disks retained thick provisioning during migration from traditional storage arrays.
• OSR Configuration: The assigned VM Storage Policy has OSR incorrectly set to 100.
• Stale Metadata: The vSAN Health Service retains cached metadata for deleted VMs or disks.

Resolution

To convert the object back to thin, force vSAN to rewrite the object by applying a temporary Thick policy, then reverting to the original Thin policy.

Note: This procedure triggers object recreations and will increase I/O operations. Perform these operations in small batches during a maintenance window or period of low cluster activity.

1. Verify Storage Policy:

   1. Navigate to vSphere Client > Policies and Profiles > VM Storage Policies.
   2. Review the assigned policy for flagged VMs. If OSR is set to 100, update it to 0.

2. Policy Toggle Workaround:

   1. Confirm no active snapshots exist (snapshots block provisioning changes).
   2. Clone the existing vSAN storage policy and edit the clone to set OSR to 100.
   3. Right-click the affected VM in the vSphere Client and select Edit Settings.
   4. Navigate to the VM Storage Policies tab and apply the new "Thick" policy to the affected VM.
   5. Wait for the task to complete, then re-apply the original "Thin" (OSR 0) policy to the VM.
   6. Monitor the vSAN backend resync task until completion.

3. Clear Stale/Ghost Alarms:

   1. Connect to the vCenter Server via SSH as root.
   2. Run the following command: service-control --restart vmware-vsan-health
   3. Navigate to vSAN Skyline Health in the vSphere Client and click RE-TEST.

Additional Information

• This Health Alarm will not check VMs deployed by ESX Agent Manager (EAM), as most of the VMs deployed are thick-provisioned by default or VMs with disabled tasks (e.g., NSX Controllers).
• With Virtual Machine disk(s) configured as "Thick provisioning", vSAN datastore utilization will be higher. See KB 396137 and VMware Blog for additional information.