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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.

In the following example, the overall objective is to shrink and optimise an existing image for random read and write operations. You can also grow disk images using the same approach.

These instructions specifically use 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 virtual machine in this case is a Windows 10 guest using the NTFS file system.

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. The disk size should be smaller than the virtual size. In this particular case, the disk size is 26.7 GiB and the virtual size 64 GiB.

# qemu-img info /var/lib/libvirt/images/windows-sparsified.qcow2

image: /var/lib/libvirt/images/windows-sparsified.qcow2
file format: qcow2
virtual size: 64 GiB (68719476736 bytes)
disk size: 26.7 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 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 size of the partition /dev/sda2 is 63G. It appears to offer the greatest scope for resizing, as the overall disk size in Step 7 is only 26.7 GiB in total.

Name       Type        VFS   Label            MBR  Size  Parent
/dev/sda1  filesystem  ntfs  System Reserved  -    50M   -
/dev/sda2  filesystem  ntfs  -                -    63G   -
/dev/sda3  filesystem  ntfs  -                -    530M  -
/dev/sda1  partition   -     -                07   50M   /dev/sda
/dev/sda2  partition   -     -                07   63G   /dev/sda
/dev/sda3  partition   -     -                27   530M  /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/sda2 listed in Step 8 is equivalent to /dev/nbd9p2 connected as a network block device. Use GNOME Disks to shrink /dev/nbd9p2 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 32G 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 32G

Step 15

Copy the source image to the target image and specify the partition to expand in the process.

# virt-resize --expand /dev/sda2 /var/lib/libvirt/images/windows-sparsified.qcow2 /var/lib/libvirt/images/windows-target.qcow2

Step 16

Confirm the actual disk size of the target image.

# qemu-img info /var/lib/libvirt/images/windows-target.qcow2

image: /var/lib/libvirt/images/windows-target.qcow2
file format: qcow2
virtual size: 32 GiB (34359738368 bytes)
disk size: 32 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 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/sda2 of the virtual device /dev/sda is now 31G in size. The overall disk size in Step 16 is now only 32 GiB in total.

Name       Type        VFS   Label            MBR  Size  Parent
/dev/sda1  filesystem  ntfs  System Reserved  -    50M   -
/dev/sda2  filesystem  ntfs  -                -    31G   -
/dev/sda3  filesystem  ntfs  -                -    530M  -
/dev/sda1  partition   -     -                07   50M   /dev/sda
/dev/sda2  partition   -     -                07   31G   /dev/sda
/dev/sda3  partition   -     -                27   530M  /dev/sda
/dev/sda   device      -     -                -    32G   -

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.