Compress virtual machine disks of Hyper-V

Recently, my virtual machine disks of Hyper-V became very large, then I decided to compress them. Here is how I did it.

1. Enter the virtual machine, and execute the following commands for each disk’s root path:

   1dd if=/dev/zero of=/path/to/disk/zero-file # Write a zero-bytes file to the root path of the a disk
   2sync
   3rm /zero-file
   

2. Shut down the virtual machine.

3. Open a PowerShell with administrator privileges, and execute the following command for each vhdx file:

   1Optimize-VHD -Path "C:\path\to\disk.vhdx" -Mode Full
   

All done.

Actually, this method can only do a little compression. If the disk is still too large, you may consider creating a new vhdx file and migrating the data to it.