When you create a virtual machine in VMware vSphere, vSphere creates a new virtual hard drive for that virtual machine. The virtual hard drive is contained in a virtual machine disk (VMDK). The disk format you choose for the new virtual hard drive can have a significant impact on performance.
You can choose one of three formats when creating a virtual hard drive:
When vSphere creates a thin-provisioned disk, it only writes a small amount of metadata to the datastore. It does not allocate or zero out any disk space. At write time, vSphere first updates the allocation metadata for the VMDK, then zeros out the block or blocks, then finally writes the data. Because of this overhead, thin-provisioned VMDKs have the lowest performance of the three disk formats.
You can use thin provisioning to over-commit disk spaces to VMs on a datastore. For example, you can put 10 VMs, each with a 50 GB VMDK attached to it, on a single 100 GB datastore, as long as the sum total of all data written by the VMs never exceeded 100 GB. Thin provisioning allows administrators to use space on datastores that would otherwise be unavailable if using thick provisioning, possibly reducing costs and administrative overhead.
When vSphere creates a Thin-provisioned lazy-zeroed disk, it allocates the maximum size of the disk to the VMDK, but does nothing else. At the initial access to each block, vSphere first zeros out the block, then writes the data. Performance of a Thin-provisioned lazy-zeroed disk is not as good a thick provisioned eager zero disk because of this added overhead.
When vSphere creates a Thin-provisioned eager-zeroed disk, it allocates the maximum size of the disk to the VMDK, then zeros out all of that space.
Example: If you create an 80 GB Thin-provisioned eager-zeroed VMDK, vSphere allocates 80 GB and writes 80 GB of zeros.
By overwriting all data in the allocated space with zeros, Thin-provisioned eager-zeroed eliminates the possibility of reading any residual data from the disk, thereby reducing possible security risks.
Thin-provisioned eager-zeroed VMDKs have the best performance. When a write operation occurs to a Thin-provisioned eager-zeroed disk, vSphere writes to the disk, with none of the additional overhead required by thin provisioned or Thin-provisioned lazy-zeroed formats.