qcow

qcow is a file format for disk image files used by QEMU, a hosted virtual machine monitor. It stands for "QEMU Copy On Write" and uses a disk storage optimization strategy that delays allocation of storage until it is actually needed. Files in qcow format can contain a variety of disk images which are generally associated with specific guest operating systems. Three versions of the format exist: qcow, qcow2 and qcow3 which use the .qcow, .qcow2 and .qcow3 file extensions, respectively.

qemu-img command allows to inspect, check, create, convert, resize and take snapshot of qcow images. The e2fsprogs command e2image also has support for generating qcow2 files to avoid the use of sparse file support.