vSphere supports two models of storage provisioning, thick provisioning and thin provisioning.

Thick provisioning

It is a traditional model of the storage provisioning. With the thick provisioning, large amount of storage space is provided in advance in anticipation of future storage needs. However, the space might remain unused causing underutilization of storage capacity.

Thin provisioning

This method contrast with thick provisioning and helps you eliminate storage underutilization problems by allocating storage space in a flexible on-demand manner. With ESXi, you can use two models of thin provisioning, array‐level and virtual disk‐level.

Thin provisioning allows you to report more virtual storage space than there is real physical capacity. This discrepancy can lead to storage over-subscription, also called over-provisioning. When you use thin provisioning, monitor actual storage usage to avoid conditions when you run out of physical storage space.