VMware ESXi uses a thick virtual disk by default when it creates a virtual disk file.. The thick disk preallocates the entire amount of space specified during the creation of the disk. For example, if you create a 10 megabyte disk, all 10 megabytes are preallocated for that virtual disk.

In contrast, a thin virtual disk does not preallocate disk space. Blocks in the VMDK file are not allocated and backed up by physical storage until they are written on the disk during the normal course of operation. A read instruction to an unallocated block returns zeroes, but the block is not backed by physical storage until it is written on the disk. Consider the following details when implementing thin-provisioned disk in your VMware environment:

• Thin-provisioned disks can grow to the full size as specified at the time of virtual disk creation, but they cannot shrink. Once you allocate the blocks, you cannot deallocate them.

• Thin-provisioned disks run the risk of overallocating storage. If storage is over-allocated, thin virtual disks can grow to fill an entire datastore if left unchecked.

• If a guest operating system needs 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 formatting process may cause the thin-provisioned disk to grow to full size. For 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 format tool in Microsoft Windows writes information to all sectors on the disk, which in turn inflates the thin-provisioned disk to full size.

Thin-provisioned disks can overallocate storage. If the storage is overallocated, thin virtual disks can grow to fill an entire datastore if left unchecked. You can use thin-provisioned disks, but you must use strict control and monitoring to maintain adequate performance and ensure that storage is not consumed to its full capacity. If operational procedures are in place to mitigate the risk of performance and storage depletion, then thin-provisioned disks are a viable option.