vSAN Capacity and Thick-provisioned VM Alarms in 2-Node Cluster

Products

• VMware vSAN

Issue/Introduction

• The vSphere Client displays the following triggered alarms in a vSAN environment:

1. vSAN online health alarm 'Thick-provisioned VMs on vSAN'
2. vSAN capacity utilization alarm 'Storage space'
3. Datastore usage on disk

• The actual consumed storage on the vSAN datastore is below the standard 80% threshold (e.g., 70% - 76%).

• The environment consists of a 2-Node vSAN cluster (commonly used for Remote Office/Branch Office or Disaster Recovery sites).

• The affected virtual machines are core management appliances, such as VMware vCenter Server, VxRail Manager, or vSphere Replication.

Environment

• VMware vSAN 7.x / 8.x

• VxRail Appliances

• 2-Node vSAN Cluster

Cause

Management appliances (vCenter, VxRail Manager, Zerto, etc.) are often deployed via OVF/OVA templates that default to a vSAN Storage Policy with Object Space Reservation (OSR) set to 100%.
In 2-node clusters with lower capacity, this "thick" reservation triggers dynamic health thresholds and static vCenter datastore alarms (defaulting at 75%) before the datastore is physically full.

Resolution

To reclaim reserved storage space and clear these alarms, update the storage policy for the affected management VMs to use thin provisioning (Object Space Reservation = 0%).

Step 1: Create a Thin Provisioning vSAN Storage Policy

1. Log in to the vSphere Client.
2. Navigate to Policies and Profiles > VM Storage Policies.
3. Locate the default policy (e.g., "vSAN Default Storage Policy"), right-click it, and select Clone.
4. Name the policy (e.g., vSAN Thin Policy) and click Next.
5. On the vSAN rules page, set Object Space Reservation to 0%.
6. Complete the wizard to save the policy.

Step 2: Apply the New Storage Policy

Best Practice: Apply this change to one virtual machine at a time to prevent high I/O latency during backend reconfiguration in a 2-node cluster.

1. Navigate to Hosts and Clusters and select the affected management VM.
2. Right-click the VM, select VM Policies, and click Edit VM Storage Policies.
3. Select the vSAN Thin Policy from the drop-down menu.
4. Click Apply to All, then click OK.
5. Monitor progress under the Monitor tab > vSAN > Resyncing Objects.
6. Once resyncing completes for the first VM, repeat these steps for any remaining management appliances.

Additional Information

• KB 326904: Health Alarm -- "Thick-provisioned VMs on vSAN" detected on vSAN-health check 

• KB 326889: vSAN Health Service - Capacity utilization - Storage space