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QEMU Copy On Write
Filename extension
qcow, qcow2
Developed byQEMU
Type of formatDisk container
Open format?Yes

.qcow and .qcow2 (QEMU Copy On Write) are disk image containers created for and used by the QEMU virtual machine emulator. QCOW containers can contain any partition table and file system(s) used by a guest operating system, it is just a regular hard drive from a guest operating systems perspective.

There are three versions of QCOW, though there are only two main ones: qcow and qcow2. The newest qcow3 format is just an extension of qcow2 and tools like qemu-img refer to qcow3 as qcow2, there is no -f qcow3 switch for specifically creating qcow3 images.

qemu-img, a tool that comes with QEMU, can be used to inspect, check, create, convert, resize and take snapshot of qcow images.

QCOW2 images can be compressed by qemu-img. QEMU can read compressed images, but it won't write compressed data. An existing .qcow2 container can be compressed by, oddly, "converting" it:

qemu-img convert -c -f qcow2 -O qcow2 \
 gentoo.qcow2 gentoo-compressed-`date --iso`.qcow2

Compressing qcow2 images will typically reduce their size by half, and in some cases as much as one third. However, the images tend to "grow back" in size over time; you may want to compress images you use on a regular basis once every three months or so.

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