I was just reading this article by Vladan about Storage VMotion. He explains how you can get your unused disk space back with Storage VMotion and moving to a Thin Provisioned disk at the same time. I agree that this is one of the best new features out there. It’s easy and effective.

However, you will need to keep in mind that although it seems like disk space is not used according to the Guest OS it might have been used in the past. (An OS usually only removes the pointer to the data and not the actual data itself.) If you d0 not zero out your disk before you do the Storage VMotion and migration to a thin provisioned disk you will be copying all the “filled” blocks. This is actually the same concept as for instance a VCB full image dump, which I addressed in the beginning of 2008.

So for optimizing migrations to Thin Provisioned disks either use sdelete by Microsoft/Sysinternals or use the “shrink” option within VMware tools. Both work fine, but keep in mind they can be time consuming. You could use sdelete to script the solution and actually zero-out every disk once a week.

http://www.yellow-bricks.com/2009/07/31/storage-vmotion-and-moving-to-a-thin-provisioned-disk/