Qemu as an emulator allow the user to enjoy the virtualization on their system without the need to install a OS separately. Qemu provides qemu-img command to create the hard disk. This command is used to create images of various formats like qcow (QEMU copy-on-write). This format allows the size of the disk image is different from the file that is physically present. It creates a compact image such that the size decreases and the image that gets produced is compressed to make it protable. For example: to create a disk image of 128 MB the following code will be written.

Qemu-img create -f qcow disk.img 128MB