Fast provisioning saves time by using linked clones for virtual machine provisioning operations.
A linked clone is a duplicate of a virtual machine that uses the same base disk as the original, with a chain of delta disks to track the differences between the original and the clone. If fast provisioning is disabled, all provisioning operations result in full clones.
A linked clone cannot exist on a different vCenter datacenter or datastore than the original virtual machine. vCloud Director creates shadow virtual machines to support linked clone creation across vCenter datacenters and datastores for virtual machines associated with a vApp template. A shadow virtual machine is an exact copy of the original virtual machine. The shadow virtual machine is created on the datacenter and datastore where the linked clone is created. You can view a list of shadow virtual machines associated with a template virtual machine. See View Shadow Virtual Machines Associated With a vApp Template.
Fast provisioning is enabled by default on organization virtual datacenters. Fast provisioning requires vCenter 5.0 and ESXi 5.0 hosts. If the provider virtual datacenter on which the organization virtual datacenter is based contains ESXi 4.x hosts, you must disable fast provisioning. See Edit Organization Virtual Datacenter Storage Settings.