When you create a virtual machine, a certain amount of storage space on a datastore is provisioned or allocated to the virtual disk files.

By default, ESX offers a traditional storage provisioning method during creation in which you estimate how much storage the virtual machine will need for its entire life cycle, provision a fixed amount of storage space to its virtual disk, and have the entire provisioned space committed to the virtual disk. A virtual disk that immediately occupies the entire provisioned space is called a thick disk. Creating virtual disks in thick format can lead to underutilization of datastore capacity, because large amounts of storage space, pre-allocated to individual virtual machines, might remain unused.

To help avoid over-allocating storage space and save storage, ESX supports thin provisioning, which lets you, in the beginning, use just as much storage capacity as currently needed and then add the required amount of storage space at a later time. Using the ESX thin provisioning feature, you can create virtual disks in a thin format. For a thin virtual disk, ESX provisions the entire space required for the disk’s current and future activities, but commits only as much storage space as the disk needs for its initial operations.