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Thick provision - Space required for the virtual disk is reserved or allocated already from the storage at time of disk creation
Lazy zeroed - Data on the disk is erased at the time of creation and it's zeroed out only when new data is written to the disk
Eager Zeroed - Disk is zeroed out at the time of creation itself and then data is written on disk
Most of us are familiar with the difference between a thin-provisioned virtual disk and a thick-provisioned virtual disk. A thick disk's blocks are allocated in VMFS when the disk is created and a thin disk's blocks aren't. Not allocating the blocks at creation allows a thin disk to consume space as it needs it.
There is, however, a third type of disk provisioning in ESX—eager zeroed thick. This disk provisioning process goes a step further than the thick process by fully allocating and zeroing out all the data inside the disk array at the time the disk is provisioned.
This is an important distinction because a regular thick disk will require a short pause as its data expands to zero out data on the array. This isn't the case in an eager zeroed disk, where the space on the array has already been provisioned and the data zeroed out.
The difference in performance between a thick disk and an Eager Zeroed thick disk is very small, but some applications, such as Microsoft Cluster Services and VMware Fault Tolerance, still require eager zeroed thick provisioned disks.
Thick Provision Lazy Zeroed:
Creates a virtual disk in a default 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 virtual machine.
Using the default flat virtual disk format does not zero out or eliminate the possibility of recovering deleted files or restoring old data that might be present on this allocated space. You cannot convert a flat disk to a thin disk
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. In contrast to the flat format, the data remaining on the physical device is zeroed out when the virtual disk is created. It might take much longer to create disks in this format than to create other types of disks.
Thin Provision:Use this format to save storage space. For the thin disk, you 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.
