Shrink qcow2 virtual disk

IT May 5, 2024

As mentioned in my post about Looking glass I run a Windows VM on my Ubuntu machine. This works perfectly fine, but the virtual disk tends to grow slowly over time, taking up space on my SSD. I'm pretty neurotic about cleaning up my storage spaces, see it as my personal "clean deisk policy", so how can I retrieve this disk space?

Following the steps of the "Manual Solution" described in the Proxmox guide linked above, you can free up unused space taken up by the VM. A summarized version is copied next:

Prepare VM

Cleanup your VM
Defragment your Windows guest's drives
Download SDelete and save it in eg. C:\windows\system32\
Run SDelete and zero free space:

sdelete -z c:

Shrink the disk

Shut down VM
Make a backup of the virtual disk

cd /var/lib/libvirt/images
cp image.qcow2 image.qcow2_backup

Either shrink the disk

qemu-img convert -O qcow2 image.qcow2_backup image.qcow2

Or shrink and compress the disk

qemu-img convert -O qcow2 -c image.qcow2_backup image.qcow2

Results

There are two ways of shrinking the disk, with and without compression, but how do they compare? What is meant by "smaller disk size" and "takes longer to shrink"?

My CPU is a Xeon W-1250 @ 3.3 GHz (4.7 GHz turbo). My SSD is a Samsung PM9A1 NVMe PCIe 4.0 x4 (my CPU only supports PCIe 3.0). In my specific case these are the numbers.