Q. What's the difference between a thick and eager zeroed thick virtual disk in ESX?

A. 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.

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