How to Reclaim Unused Space in Dynamic VHDX in Windows Server Hyper-V? #HyperV

To reclaim unused space in a dynamic VHDX file, you can use the “Shrink” option in the Hyper-V Manager or use the “Optimize-VHD” cmdlet in PowerShell.

In this article, we will show you how to reclaim unused disk space in Dynamic VHDX in Windows Server Hyper-V.

Introduction

The other day I had a request to send a Virtual Machine over the continent.

Well, this is very easy, but what if the size of the VM is large and you have limited Internet bandwidth?

So without further ado, let’s jump into the scenario:

The guest OS is Windows Server 2012 R2 with 50GB dynamic VHDX. This VM is used by the Dev department for Webserver testing purposes. They do copy large files, delete, etc…

The VM is Generation 1 and has an IDE attached to VHDX.

If we look inside the VM we can see that ~15GB of space is being used which is basically the OS bits, updates, and small web application:

However, if we look at the host, we can see that this VM is occupying 45GB of disk space, this is odd

In Windows Server 2012 and above, Microsoft introduced the TRIM/UNMAP feature. When a large file is deleted from the file system of a VM guest operating system, the guest operating system sends a file delete request to the virtual machine’s virtual hard disk file. The VM’s VHD or VHDX file tunnels the UNMAP request to the class driver stack of the Windows Hyper-V host. So in the scenario above, the disk would initially expand from 15GB to 45GB but when the large files are deleted inside the guest, this would theoretically (automatically) be able to drop back down the VHDX file to ~15GB size.

Why I said theoretically is because the hardware needs to specifically support this. So you’ll need to check with the storage vendor to know if this will work or not.

In my scenario, I am running the Virtual Machine locally on DAS storage, therefore the TRIM/UNMAP feature won’t work automatically.

Reclaim Unused Space in Dynamic VHDX

So how can we reclaim back the unused space?

With PowerShell, of course…

Let’s fire up PowerShell within the guest OS and force UNMAP using the following cmdlet:

Optimize-Volume –DriveLetter C –ReTrim –Verbose

Shut down the VM and look again at the size of the dynamic VHDX file. It shrinks back to the original file size of ~15GB.

If you like to see TRIM/UNMAP on a Dell Compellent SAN in action, my fellow MVP Didier Van Hoye has recorded a screencast demo.

In conclusion, Trim requires trim-compatible hardware and also requires Windows Server 2012 or later release on Hyper-V hosts and guests.

Update: Storage Spaces Direct (Azure Stack HCI) supports UNMAP/Thin provisioning, so if your Virtual Machines are stored on Storage Spaces volumes, you will get the benefit of UNMAP automatically without any trim-compatible hardware.

In the upcoming post, we will see how can we reclaim unused space in dynamic VHDX in the older guest OS.

Hope this helps.

Until then… enjoy your day!

Cheers,
/Charbel