Howto Shrink a Thin Provisioned Virtual Disk (VMDK)

Thin provisioned disks are a great feature to save capacity as you virtual machines filesystem will never use the full capacity. I do not know a single system where you do not have at least 10GB of free space for OS disks. I am not considering databases, applications or fileservers which will grow constantly. Having thin provisioned disk is usually no longer a performance problem so it is a valid design choice even in production.

A common issue with thin disks is that the size will grow when required, but never shrink. When you require the capacity only once you might want to get it back from the virtual machine. This post describes how to reclaim unused space from the virtual machine.

Virtual Machine Preparation (Windows)

Windows does not automatically zero deleted blocks. Microsoft provides a tool that can zero blocks after while deleting a file or zero out the entire free space. This is required to reclaim space back from the virtual disk.

  1. Download SDelete
  2. run sdelete.exe -z [Drive]

Wait a couple of minutes until the process is finished. Please note that you virtual disk file (VMDK) will grow to the full size during the process.

Virtual Machine Preparation (Linux)

Linux does not zero deleted blocks too. There are various tools available to create zeroed blocks. The best known tool is dd which should be available on all systems.

  1. Identify free space with df
  2. Fill the free space with dd
vma:/mnt/data # df -h
vma:/mnt/data # dd bs=1M count=8192 if=/dev/zero of=zero

This will zero 8GB of the available 8.2GB (1MB Blocksize * 8192 = 8GB). Please note that you virtual disk file (VMDK) will grow to the full size during the process.dd-fill-free-space-with-zero

Shrink VMDK File

  1. Power off the Virtual Machine, or disconnect the virtual Disk you want to shrink
  2. Connect to the ESXi Host with SSH
  3. Navigate to the Virtual Machine Folder
  4. Verify disk usage with du
  5. Run vmkfstools -K [disk]
  6. Verify disk usage with du
root@esx3:/vmfs/volumes/ds1/vma $ du -h vma_1-flat.vmdk
7.9G vma_1-flat.vmdk

root@esx3:/vmfs/volumes/ds1/vma $ vmkfstools -K vma_1.vmdk
vmfsDisk: 1, rdmDisk: 0, blockSize: 1048576
Hole Punching: 25% done.

root@esx3:/vmfs/volumes/ds1/vma $ du -h vma_1-flat.vmdk
1.9G vma_1-flat.vmdk


  1. thank you! an important part that my eyes had first bypassed is that we hole-punch the small .vmdk, not the -flat.vmdk

  2. Excellent post! I am going to do this on a volume that is 1.2TB - any ideas how long that will take? I need to build a communication for the environment on how long the VM will be down. Thanks!

  3. Hi, Josh.
    You can do it completely without downtime with Extrasphere disks (not VM) migration to another datastore even on free licensed ESXi host.

  4. Thanks for sharing this useful info. I have a question after applying these steps. flat.vmdk size has been decreased but VM Provisioned Storage is still showing the same size as original flat.vmdk. Please let me know how I can reduce this size as well? Any help is highly appreciable. Than You.

  5. Important addition regarding the Linux preparation step: the "dd" command creates a file named "zero" inside the current directory with the given size (8GB in your example) because of the parameter "of=zero". Therefore not only the virtual disk (VMDK) grows up but also the virtual drive inside the guest gets used nearly 100%. So you should mention that the file "zero" must be deleted after shrinking to free up the filled space inside the guest!

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