VM Disk provisioning policies

When you perform certain VM management operations, such as creating a virtual disk, cloning a VM to a template, or migrating a VM, you can specify a provisioning policy for the virtual disk file.

NFS datastores with Hardware Acceleration and VMFS datastores support the following disk provisioning policies. On NFS datastores that do not support Hardware Acceleration, only thin format is available.

You can use Storage vMotion to transform virtual disks from one format to another.

Thick Provision Lazy Zeroed :  Creates a virtual disk in a thick format. Space required for the virtual disk is allocated when the virtual disk is created. Data remaining on the physical device is not erased during creation, but is zeroed out on demand at a later time on first write from the VM.

Thick Provision Eager Zeroed: A type of thick virtual disk that supports clustering features such as Fault Tolerance. Space required for the virtual disk is allocated at creation time. The data remaining on the physical device is zeroed out when the virtual disk is created. It takes longer time to create disks in this format than to create other types of disks.

Thin Provision: Use this format to save storage space. Thin disk, provision as much datastore space as the disk would require based on the value that you enter for the disk size. However, the thin disk starts small and at first, uses only as much datastore space as the disk needs for its initial operations. By implementing thin provisioned disks, you are able to over-allocate storage. If storage is over allocated, thin virtual disks can grow to fill an entire datastore if left unchecked.

In order for a guest operating system to make use of a virtual disk, the guest operating system must first partition and format the disk to a file system it can recognize. Depending on the type of format selected within the guest operating system, the format may cause the thin provisioned disk to grow to a full size.

example, if you present a thin provisioned disk to a Microsoft Windows operating system and format the disk, unless you explicitly select the Quick Format option, the Microsoft Windows format tool writes information to all of the sectors on the disk, which in turn inflates the thin provisioned disk.

Thanks to VMware, Information is from the white paper provided by VMware.