Q. How can I convert a thin-provisioned disk to thick, or a thick-provisioned disk to thin, in ESX 4.0?

A. In my previous FAQ, I talked about the differences between thick-provisioned disks and thin-provisioned disks. Thin-provisioned disks eliminate wasted disk space, but at the cost of a tiny bit of extra resource use. They also require quite a bit more monitoring to prevent the very nasty situation where your physical disks run out of space.

In your virtual administration, you may, from time to time, find the need to convert a virtual machine's (VM’s) disk from thick-provisioned to thin or vice versa. Going from thin to thick is quite easy (if you have the available storage space). Within the vSphere client, select the VM in the inventory then click its Summary tab. From there, double-click the Datastore to open the Datastore Browser console. Navigate to the correct VM’s folder and right-click the VMDK file you want to convert. Select Inflate to convert the disk.

Going from thick to thin requires a very different process, because reducing a disk’s size tends to be a more complex operation than simply adding more disk space. One way is to use Storage VMotion to create a new disk that's thin provisioned. Right-click the VM and select Migrate. When prompted for storage, select Thin Provision.

A second way, which is documented in VMware KB article 1013052, involves the use of the vmkfstools command to clone the disk and convert it to thin format. Use the syntax

vmkfstools –i  -d thin