Using libvirt we can grow the size of partitions of a virtual drive without having to resize the partitions manually. This spares a lot of time and annoyances with having to boot a Live CD and then expand the partition by hand.

The two tools necessary for this are: virt-filesystems and virt-resize, both of which can be found on Debian by installing the package libguestfs-tools:

aptitude install libguestfs-tools

Using virt-filesystems we can list he filesystems inside an image. Suppose that we have an image file called first.img stored in /var/lib/libvirt/images/ that is 8GB large and we would like to expand the storage and the filesystem to 50GB. We would first issue:

virt-filesystems --long -h --all -a /var/lib/libvirt/images/first.img

which would something like:

Name       Type        VFS   Label  MBR  Size  Parent
/dev/sda1  filesystem  ext4  -      -    8.0G  -
/dev/sda1  partition   -     -      83   8.0G  /dev/sda
/dev/sda   device      -     -      -    8.0G  -

This means that we have a filesystem inside the image as a partition named /dev/sda1 that has 8GB as expected.

Now we create a new image second.img based on the fist image using truncate:

truncate -r /var/lib/libvirt/images/first.img /var/lib/libvirt/images/second.img

Next, we expand the new image (second.img) to 50GB using truncate':

truncate -s +42GB /var/lib/libvirt/images/second.img

Finally, the filesystem inside the image that we found out lies at /dev/sda1 has to be expanded to fill the remaining space:

virt-resize --expand /dev/sda1 /var/lib/libvirt/images/first.img /var/lib/libvirt/images/second.img

and we are done.