Qcow2 is the default virtual disk storage format used by Qemu (qcow stands for qemu copy-on-write). This image format makes use of thin provisioning, so, after we initially set the maximum virtual size of a disk, space is actually allocated only when used, but not made available back to the host when freed.
qemu
Access and modify virtual machines disk images with libguestfs tools
In a previous article, we saw how to create kvm virtual machines from the command line; in this tutorial, instead, we learn how to access and modify virtual machines disk images, using some utilities which are part of the libguestfs package on the most commonly used Linux distributions. Those tools let us perform a variety of tasks.