When you create a new virtual machine in VMware vSphere, when you creating the disk, you are asked what kind of disc you want.
There are three disk options with different characteristics.
Thin provision. The disk is created of the desired sizes but in the storage the real space is equal to the content data size. For example, I create a 100 GB hard drive but inside there are 20 GB of data. The size of disk in the storage will be 20 GB. In this configuration we get lower write performance.
Thick provision lazy zeroed. The disk created has storage dimensions equal to those which are desired, but the blocks within are initialized immediately before being written. Average performance.
Thick provision eager zeroed. The disk has the same dimensions as that are desired but the blocks within are initialized at creation time. Performance is better.