VirtualBox/KVM: Reduce VM sizes

There are two utilities that can help discard unused blocks so that VMs can be shrunk.

zerofree finds unused blocks with non-zero content in ext2, ext3 and ext4 filesystems and fills them with zeros. The volume can’ be mounted which makes the process of running it a bit convoluted.

fstrim will discard unused blocks on a mounted filesystem. It is best and preferred when working with SSD drives and thinly provisioned storage. It will work with more filesystems, and it won’t hammer your SSD with unnecessary writes.

It is recommended to use fstrim and only use zerofree if unavoidable.

CentOS 7/8


# fstrim -va

zerofree (ext2, ext3, ext4)

# yum install epel-release
# yum install zerofree

Press e on GRUB menu
Go to line that starts with 'linux'
Add init=/bin/bash

[Find which disk to trim]
# df
# zerofree -v /dev/mapper/centos_centos7-root

[Shutdown machine]

zerofree (xfs)

# yum install epel-release
# yum install zerofree

Press e on GRUB menu
Go to line that starts with 'linux'
Change ro to rw
Add init=/bin/bash

[Find the partition/filesystem to trim]
# df

[Fill the filesystem with zeros. This will work with any filesystem but it will write a lot of data on your drives.]
# dd if=/dev/zero of=/tmp/dd bs=$((1024*1024)); rm /tmp/dd
# sync
# exit

[Shutdown machine]

Debian 9/10


[Debian 9]
# fstrim -va

[Debian 10]
# fstrim -vA


# apt install zerofree

Press e on GRUB menu
Go to line that starts with 'linux'
Add init=/bin/bash

[Find disk to trim]
# df
# zerofree -v /dev/sda1

[Shutdown machine]

Ubuntu 18.04/20.04

[Ubuntu 18.04]
# fstrim -va

[Ubuntu 20.04]
# fstrim -vA

Be aware that if you are using ZFS on Ubuntu (or any other distro) the above commands won’t work. In fact, it will generate a lot of extra writes on the filesystem.

Just ensure that ZFS is using compression, or avoid it in the guest system.

Reducing the image size


[List all disks]
$ vboxmanage list hdds

[Just the paths]
$ vboxmanage list hdds | grep  'Location.*.vdi' | awk '{$1=""}1'

[Compress one image]
$ vboxmanage modifymedium disk --compact /home/user/Virtualbox/Kali-Linux-2021.1/Kali-Linux-2020.4-vbox-amd64-disk001.vdi

[List all images path]
$ vboxmanage list hdds | grep  'Location.*.vdi' | awk '{$1=""}1' | sed 's/^ /"/;s/$/"/'

I wish I knew the syntax to automatise compressing all the images with one line. I might revisit it in the future with a script.


# qemu-img convert -O qcow2 originalfile compressedfile

I have a script to do all of the files in one go:


# All images
for file_name in `ls -1 *.cow2`

	echo ==================
	echo Image: $file_name
	echo -n Old `qemu-img info $file_name | grep 'disk\ size'` ; echo
	mv $file_name $file_name.tmp
	qemu-img convert -O qcow2 $file_name.tmp $file_name
	rm $file_name.tmp
	echo -n New `qemu-img info $file_name | grep 'disk\ size'` ; echo
	echo ==================

KVM/QEMU: Reduce the size of your VM files

When virt-manager creates new images by default they will take the full size assigned to the virtual storage on the VM.

If your host system is using a filesystem with built-in compression like ZFS or BTRFS it won’t affect the amount of free space left on the host.

But if you aren’t or want to copy the VM to another host, then you would like to reduce its size.

You can remove unneeded files/defragment/zero the empty space of the filesystem depending on the OS the VM is running before to help the file reduction.

For comparison purposes you can check the internal disk size before the optimisation and also after with the following command:

# qemu-img info linux-vm.qcow2
image: linux-vm.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 1.6G
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

Stop the VM and then process the VM file:

# qemu-img convert -O qcow2 linux-vm.qcow2 linux-vm-compressed.qcow2

Depending on the size of the image and the machine running the command it can be time consuming, but the size reduction is worth it.

An alternative way of reducing the VM size is by using virt-sparsify.

Install the libguestfs-tools package.

[CentOS 7]
yum install libguestfs-tools

[CentOS 8]
dnf install libguestfs-tools

# apt install libguestfs-tools

Stop the VM and then process it with:

# virt-sparsify --in-place linux-vm.qcow2

The –in-place option won’t create a new file and will compress the existing one.

AMD/Gigabyte: Enable hardware acceleration for virtualisation in the BIOS

In the Gigabyte BIOS you need to activate a series of settings on different sections in order for KVM to be able to use hardware acceleration.

  • M.I.T. -> Advanced Frequency Settings -> Advanced CPU Core Settings -> SVM Mode set to Enable
  • Chipset -> IOMMU set to Enabled

Save settings and reboot.

Run the KVM check to see if the system is capable or running hardware accelerated virtualisation.


If you still fail the check review your BIOS/motherboard documentation to activate the correct setting.