Storage VMotion and moving to a Thin Provisioned disk

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.

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    1. wpatton says


      Thank you very much for clarifying this. Great information. It was something we were just debating internally.

    2. wpatton says


      Are you aware of any way to see size/blocks filled to allow you to verify this against the actual used space within the VM? Migrating to Thin Provisioning and then shrinking the disk should do it, but before we do this we are tasked with estimating savings.

      Thank you.

    3. Tom says

      Where can I find more information on this??
      After we get vsphere installed (waiting for SAN to get on HCL) I want to change as much as possible to thin-provisioned disks and be more economical with storage space.

    4. Jason says

      I read the following tip which states
      “Avoid doing operating systems disk defragmentations on VMs with thin disks. This will cause the VM disk file to grow quickly which may fill up your data store before you can react to any alarms that the process may trigger.” Found here:

      Your script does a defrag prior to the sdelete. Any suggestions regarding this?

    5. AQU says

      We have created a scheduled task initiating a Storage vMotion while converting to a Thin Disc (Sdelete had already run). All goes fine untill in the end thse disc is still thick and no space has been gained.

      When we do the same manually with the same useraccount, all goes fine. What’s wrong?

    6. says

      Hi Duncan,

      I wonder if VAAI on EQL will significantly speed up the sdelete process on thin disk before storage vmotion to reclaim space?