Shrink, optimise and expand an existing QCOW2 image

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 which 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.

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.