There are two different types of disk provisioning which are thick (or preallocated) and thin (or dynamically). Both of these related with disk size.
Thick (or preallocated)
While we creating thick provisioning disk, whole defined space allocated on physical disk, for examples we planning to allocate 80GB disk space to new virtual machine that 80GB disk space fully consumed on your physical drives.
Thin (or dynamically)
In thin disk provisioning system, It will consume the space for physical disk as per grow size only. For examples, if you create a 50GB disk but only use 10GB of that disk, the actual disk consumption on your physical drives will be 10GB.
In the condition that you already created the the VM machine with disk provisioning type is thin. But you want to change the provisioning type from thin to thick. In this post i will show you the simple steps to change the disk provisioning type. I have tested this in the ESXi 4.1 host server. Before follow this steps, make sure that you are appointed VMware administrator.
1. Login as root in ESXi host if you dont have VCenter Server.
2. Browse the datastore
3. Select the VM machine that you want to convert the disk provisioning type and double click the folder.
4. Find the .vmdk then right click.
5. Click Inflate to change it to thick provisioning type. This will take sometimes to completed.
6. SSH using root to ESXi host and find the .vmdk that your already inflated.
7. To change format, you should do something like this:
vmkfstools -i CentOS5.5_mythin.vmdk -d zeroedthick CentOS5.5_mythick.vmdk
CentOS5.5_mythin.vmdk = your old dmdk
CentOS5.5_mythick.vmdk = your new vmdk