How to convert VM hard drive from Thick to Thin Provisioning

Using Thin Provisioning for virtual disks of your VM can save you a lot of useful space on your datastore and significantly speed up maintenance tasks that involve copying or moving of the VMDK files. Apparently, switching from one provisioning format to another can be done easily via Clone Virtual Machine wizard, where Storage tab allows you to specify the virtual disk format for the cloned VM. There you can select either to keep the same disk format as source VM had or choose another one, including Thin Provision. However, the choice of Thin Provision disk format will be silently ignored when the source format is Thick Provisioned and both source and target are located in the same datastore. This behavior is somehow understandable when you think about the underlying implementation, but not user friendly.

So, how can you change the provisioning type of your VM from Thick to Thin anyway? Here is your algorithm.

  • Do you have more than one datastore available? You can migrate your VM to another datastore, modifying the virtual disk provisioning mode on the way, and then migrate it back if needed. This way is covered in details in the second part of KB2014832 article on VMware Knowledge Base.
  • Do you have only one datastore or all your datastores are under the same Storage DRS? You can create a clone of your VM with the help of vCenter Converter Standalone. This tool runs on your desktop and allows you to convert one VM format into another, changing most of the VM setting on the way. This includes change of disk provisioning type from Thick to Thin even when both source and destination are located on the same datastore.
  • Do you prefer the hard and risky ways? If so, dealing with vmkfstools if for you 🙂

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