Virtual machine disk provisioning type changes unexpectedly

Products

• VMware vSphere ESXi

Issue/Introduction

This article clarifies when virtual disk provisioning type can change on ESX hosts. (Disk provisioning type is sometimes called disk allocation type.)

Note: Virtual disk provisioning types are "thin", "thick" or "lazy zeroed thick" (LZT), and "eager zeroed thick" (EZT). 
For more details, refer: Virtual Disk Provisioning Policies

Symptoms:

• After creating, migrating or cloning a virtual disk, you notice the provisioning type of the disk does not match what you selected in the UI or specified in the API call.
• After a VM has been running for a period of time, you notice the provisioning type of a disk attached to that VM has changed from thin or thick to eager zeroed thick.
• After snapshotting, restoring, or backing up a VM, you notice the provisioning type of a disk attached to that VM has changed.
• After inflating, growing, shrinking, encrypting, or otherwise modifying a virtual disk, you notice its provisioning type has changed. 

Note: The current provisioning type of a disk can be seen in the "Edit Settings" tab or the "VM Hardware" tile in the HTML5 UI.

Environment

VMware vSphere 7.0.x
VMware vSphere ESXi 6.7
VMware vSphere ESXi 6.5

Cause

In general, virtual disk provisioning type is a mutable property. It can change without warning and without manual intervention. There is no guarantee it will stay constant.

• Such unexpected changes are especially common when NFS datastores are involved.
• Disk provisioning type should be seen as a creation hint. VMware software will make a best-effort attempt to create and preserve a disk as the requested type but this sometimes isn't possible.
• The only exception is for shared (multiwriter) disks on a VMFS datastore that are of eager zeroed thick (EZT) provisioning type. A shared disk on VMFS requires EZT provisioning, so VMware guarantees it will stay EZT as long as it stays on a VMFS datastore.

Resolution

This is an expected behavior.

• It is not a bug and an issue should not be filed unless a specific VM operation fails with an error code because a virtual disk provisioning type has changed.
• The only known situation where a change in provisioning type can cause a failure is discussed above:
  • A shared disk on a VMFS datastore must remain Eager Zeroed Thick or it will fail to open.