A virtual disk image is a block device in a file. There are a number of different disk image formats to choose from when setting up a virtual machine. QEMU Copy On Write version 2 (QCOW2) is the default virtual disk image format for the Quick Emulator (QEMU). Features such as thin provisioning, snapshots and compression make QCOW2 one of the most versatile virtual disk formats available.
These instructions specifically target Debian 12 with a GNOME desktop as the host, but they should also be applicable to other Linux distributions such as Ubuntu or Linux Mint. The guest in this particular example is a Windows 11 virtual machine that has run out of space.
The overall objective is to shrink and optimise the 64 GiB disk image for random read and write operations before expanding it to a desired size of 128 GiB.
With thanks to Fam Zheng.
Before you begin
Shut down the virtual machine and delete all existing snapshots from the image file.
Never modify images currently in use by a running virtual machine.
Step 1
On the host, install the necessary tools for working with virtual disk images.
$ sudo apt-get install --yes libguestfs-tools gnome-disk-utility
Step 2
Only root can access the host directory /var/lib/libvirt/images. Use the following command to obtain the necessary privileges.
$ sudo su
Step 3
Continue by creating a directory in which to keep your virtual machine backups.
# mkdir /var/lib/libvirt/backups
Step 4
Now create a backup of the virtual machine with the name windows by copying its QCOW2 image file to the backups directory.
# cp /var/lib/libvirt/images/windows.qcow2 /var/lib/libvirt/backups/windows-backup.qcow2
Step 5
Sparsify the image file to convert any free space within the disk image to free space on the host.
# virt-sparsify --in-place /var/lib/libvirt/images/windows.qcow2
Step 6
Rename the sparsified image file.
# mv /var/lib/libvirt/images/windows.qcow2 /var/lib/libvirt/images/windows-sparsified.qcow2
Step 7
Check the disk size of the sparsified image file.
# qemu-img info /var/lib/libvirt/images/windows-sparsified.qcow2
The disk size should be smaller than the virtual size. In this particular case, the disk size is 33.7 GiB and the virtual size 64 GiB.
image: /var/lib/libvirt/images/windows-sparsified.qcow2
file format: qcow2
virtual size: 64 GiB (68719476736 bytes)
disk size: 33.7 GiB
cluster_size: 65536
Format specific information:
compat: 1.1
compression type: zlib
lazy refcounts: true
refcount bits: 16
corrupt: false
extended l2: false
Step 8
Determine which partition to resize by obtaining more detailed information about the contents of the sparsified disk image.
# virt-filesystems --long -h --all -a /var/lib/libvirt/images/windows-sparsified.qcow2
On the virtual device /dev/sda, the partition /dev/sda3 is equivalent to the Local Disk (C:) of the Windows 11 virtual machine.
Name Type VFS Label MBR Size Parent
/dev/sda1 filesystem vfat - - 96M -
/dev/sda3 filesystem ntfs - - 63G -
/dev/sda4 filesystem ntfs - - 768M -
/dev/sda1 partition - - - 100M /dev/sda
/dev/sda2 partition - - - 16M /dev/sda
/dev/sda3 partition - - - 63G /dev/sda
/dev/sda4 partition - - - 768M /dev/sda
/dev/sda device - - - 64G -
Step 9
Load the network block device (NBD) kernel module.
# modprobe nbd max_part=8
Step 10
Connect the sparsified image.
# qemu-nbd --connect=/dev/nbd9 /var/lib/libvirt/images/windows-sparsified.qcow2
Step 11
The partition /dev/sda3 listed in Step 8 is equivalent to /dev/nbd9p3 connected as a network block device. Use GNOME Disks to shrink /dev/nbd9p3 to its Minimal Size.
Use a graphical utility to minimise the risk of introducing errors.
Step 12
Disconnect the resized image.
# qemu-nbd -d /dev/nbd9
Step 13
Unload the NBD kernel module.
# modprobe -r nbd
Step 14
Create a target image larger than the resized source image. In this example, the size of the target image is 128G and its format QCOW2 with full preallocation and a cluster size of 2M.
# qemu-img create -f qcow2 -o preallocation=full -o cluster_size=2M /var/lib/libvirt/images/windows-target.qcow2 128G
Step 15
Copy the source image to the target image. Specify the correct partition which to expand in the process.
# virt-resize --expand /dev/sda3 /var/lib/libvirt/images/windows-sparsified.qcow2 /var/lib/libvirt/images/windows-target.qcow2
Step 16
Confirm the size of the target image.
# qemu-img info /var/lib/libvirt/images/windows-target.qcow2
The overall disk size is now 128 GiB in total.
image: /var/lib/libvirt/images/windows-target.qcow2
file format: qcow2
virtual size: 128 GiB (137438953472 bytes)
disk size: 128 GiB
cluster_size: 2097152
Format specific information:
compat: 1.1
compression type: zlib
lazy refcounts: false
refcount bits: 16
corrupt: false
extended l2: false
Step 17
Obtain more detailed information about the contents of the target disk image.
# virt-filesystems --long -h --all -a /var/lib/libvirt/images/windows-target.qcow2
The partition /dev/sda3 of the virtual device /dev/sda is now 127G in size.
Name Type VFS Label MBR Size Parent
/dev/sda1 filesystem vfat - - 96M -
/dev/sda3 filesystem ntfs - - 127G -
/dev/sda4 filesystem ntfs - - 768M -
/dev/sda1 partition - - - 100M /dev/sda
/dev/sda2 partition - - - 16M /dev/sda
/dev/sda3 partition - - - 127G /dev/sda
/dev/sda4 partition - - - 768M /dev/sda
/dev/sda device - - - 128G -
Step 18
Rename the target image file.
# mv /var/lib/libvirt/images/windows-target.qcow2 /var/lib/libvirt/images/windows.qcow2
All done!
You can also modify format specific options for an existing image without having to create a target disk image. Or alternatively expand into a target image that uses a format compatible with other hypervisors, such as RAW, VMDK, VDI, VHD, VHDX or QED.
