Some introduction about Reserve Space which belonged to VAAI NAS Primitives

Reserve Space
In previous versions of vSphere, only a thin VMDK could be created on NAS datastores. This new reserve space
primitive enables the creation of thick VMDK files on NAS datastores, so administrators can reserve all of the
space required by a VMDK, even when the datastore is NAS. As shown in Figure 1, users now have the ability to
select lazy-zero or eager-zero disks on NAS datastore.