How to cleanup and shrink disk space usage of a Windows KVM virtual machine

We still need Windows VMs (sadly, for a few tools we’re trying to get rid of), and my VM grew so much that the image was up to 60Gb. With my laptop only having a 256Gb SSD, it was getting pretty crowded. So I set up to cleanup the Windows image and shrink it down as much as possible, and I managed to get it down to 13Gb.

Since I’m not very familiar with Windows, I leveraged the knowledge of the Internet and started cleaning my system using the tips from this article: I ran CCleaner, removed old files, uninstalled unused software. Then I went on to the “not obvious” ways to free space. I opened an administrator console and proceeded to remove shadow copies:

vssadmin delete shadows /for=c: /all

and I consolidated the Service Pack on disk, to get rid of a lot of backups from C:\windows\winsxs\:

dism /online /cleanup-image /spsuperseded

there’s a few more things you can do to save space in that directory, especially if you run Windows 8.1, Server 2012 or newer, it’s worth checking this Microsoft Technet article.

Once I cleaned up as much space as possible, I ran the Windows Defrag utility to cluster up the remaining data and then went on to fill the rest of the disk with zeroes. Think of it like doing dd if=/dev/zero of=/zero.img: you’re creating a file containing only zeroes, so that those clusters will result “empty” during the shrinking.

On Windows, the recommended tool to zero-fill your disk seems to be SDelete. I ran it as administrator in a cmd console:

sdelete -z c:

This took a long time. Hours. Best thing would probably have been to run it overnight: learn from my mistakes! :)

Note: if you have a thin disk (for example a qcow2 image), filling it up with zeroes will actually consume space on the host, up to the maximum size of the virtual disk. In my case, the image grew from a bit more than 60G to 200G. A necessary, and temporary, sacrifice.

ls -l /var/lib/libvirt/images/
-rw-r--r-- 1 root root 200G 31 dic 16.34 win7_orig.img

After SDelete finished running (and syncing to disk), I shut down the VM and prepared for the next step: shrinking the actual disk image. Thankfully, qemu-img allows you to convert to the same format. This will discard any empty cluster (remember? we filled them with zeroes, so they are empty!).

In my case, I ran two processes in parallel, because I wanted to see how much of a difference it would make to have a compressed image versus a non-compressed image, as suggested by this Proxmox wiki page:

cd /var/lib/libvirt/images/
qemu-img convert -O qcow2 win7_nocomp.img win7_orig.img &
qemu-img convert -O qcow2 -c win7_compress.img win7_orig.img &
watch ls -l

This process didn’t take too long, less than one hour, and the result was pretty interesting:

ls -l /var/lib/libvirt/images/
-rw-r--r-- 1 root root  13G  1 gen 18.13 win7_compress.img
-rw-r--r-- 1 root root  31G 31 dic 19.09 win7_nocomp.img
-rw-r--r-- 1 root root 200G 31 dic 16.34 win7_orig.img

The compressed image is less than half the non-compressed one, but you’ll use a bit more CPU when using it. In my case this is completely acceptable, because saving disk space is more important.


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