Thick provisioning is reserved or pre-allocated storage space. Thick provisioning protects systems by allowing them to function even if the vSAN datastore is full because the space is already reserved. For example, suppose you create a 10GB virtual disk with thick provisioning. In that case, the full amount of virtual disk storage capacity is pre-allocated on the physical storage where the virtual disk is created and consumes all the space allocated to it in the datastore. It won't allow other VMs to share the space from the datastore. A thin-provisioned virtual disk consumes the space that it needs initially and grows to the data space demand used in the datastore.