QCOW2
Also known as: .qcow2 file, QEMU Copy On Write, QEMU disk image
QCOW2 (QEMU Copy-On-Write, version 2) is the native virtual-disk format for QEMU and KVM. A single .qcow2 file holds a virtual machine’s entire disk and supports features like snapshots, compression, and growing on demand instead of reserving full size up front.
- Native disk format for QEMU and KVM
- Grows on demand; supports snapshots and compression
- Convertible to VDI, VMDK, and VHD
What QCOW2 does
A .qcow2 file is a virtual hard drive used by the QEMU and KVM virtualization stack common on Linux. The "copy-on-write" design lets it allocate space only as the VM writes data, so the file grows on demand rather than claiming its full size immediately.
QCOW2 also supports built-in snapshots (point-in-time saves of the VM), optional compression, and encryption — features that make it more capable than a plain raw disk image.
QCOW2 vs other virtual disks and storage
QCOW2 is the QEMU/KVM equivalent of VDI (VirtualBox), VMDK (VMware), and VHD (Microsoft); tools like qemu-img convert between these formats.
Because it grows with use and can store multiple snapshots, a .qcow2 can become very large. Deleting old snapshots, or running a compaction/convert pass, reclaims space taken by data no longer needed inside the VM.