VMware: Thin Provisioning Space Reclamation

Thin provisioning has been a great feature for virtual disk. You get the effect of using more physical resources than are actually available, yet you use less physical storage and spend less money on storage. It’s pretty easy to see the benefits. Often associated with large disk storage systems such as SANs and storage virtualization systems, thin provisioning allows enough space to be easily allocated to servers when that space is needed. 

Over allocation of storage in a thin provisioning environment allows a server to view more storage capacity than has actually been physically reserved for the server. The storage gets used only when needed by an application. This over-allocation is a key benefit of thin provisioning.

But often, data is written on a thin-provisioned drive and then deleted. This can happen at the Windows OS level, rather than at the hardware level. This occurs with thin-provisioned virtual disk drives. And in this situation, there’s a good chance you want that space back. But you typically can’t get the space back — it’s not available to use because the blocks that were previously written need to be zeroed out first. 

Thin provisioning deleted space can be reclaimed with a few different ways, eg. PerfectDisk Zero Fill, vmkfstools and SDelete.

References:

1. Virtual Disk Provisioning Policies
http://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.vm_admin.doc_50%2FGUID-4C0F4D73-82F2-4B81-8AA7-1DD752A8A5AC.html

2. Thin Provisioning