qcow2, raw, and allocation behavior

Disk format affects allocation, snapshot support, portability, and failure behavior. The filesystem, storage backend, and caching mode matter as much as the filename suffix.

Inspect image truth

qemu-img info --backing-chain /var/lib/libvirt/images/guest01.qcow2
du -h /var/lib/libvirt/images/guest01.qcow2
du -h --apparent-size /var/lib/libvirt/images/guest01.qcow2

qcow2 supports sparse allocation and internal features such as snapshots and compression. raw is structurally simple and can perform well, especially on storage that already supplies snapshots and thin provisioning.

Create explicitly

qemu-img create -f qcow2 -o preallocation=metadata /var/lib/libvirt/images/guest01.qcow2 80G
qemu-img check /var/lib/libvirt/images/guest01.qcow2

Never run modifying qemu-img operations against an image actively owned by QEMU unless the operation is specifically documented as safe through the running block layer.