qcow is a

file format A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary or free. Some file formats ...

for

disk image A disk image, in computing, is a computer file containing the contents and structure of a disk volume or of an entire data storage device, such as a hard disk drive, tape drive, floppy disk, optical disc, or USB flash drive. A disk image is us ...

files used by

QEMU QEMU is a free and open-source emulator (Quick EMUlator). It emulates the machine's processor through dynamic binary translation and provides a set of different hardware and device models for the machine, enabling it to run a variety of guest ...

, a hosted virtual machine monitor. It stands for "QEMU

Copy On Write Copy-on-write (COW), sometimes referred to as implicit sharing or shadowing, is a resource-management technique used in computer programming to efficiently implement a "duplicate" or "copy" operation on modifiable resources. If a resource is dupl ...

" 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 An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also inc ...

. 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 e2fsprogs (sometimes called the e2fs programs) is a set of utilities for maintaining the ext2, ext3 and ext4 file systems. Since those file systems are often the default for Linux distributions, it is commonly considered to be essential software. ...

command e2image also has support for generating qcow2 files to avoid the use of sparse file support.

Features

One of the main characteristics of qcow

s is that files with this format can grow as data is added. This allows for smaller file sizes than raw disk images, which allocate the whole image space to a file, even if parts of it are empty. This is particularly useful for

file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...

s that do not support

sparse file In computer science, a sparse file is a type of computer file that attempts to use file system space more efficiently when the file itself is partially empty. This is achieved by writing brief information ( metadata) ''representing'' the empty blo ...

s, such as

FAT32 File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by c ...

. The qcow format also allows storing changes made to a read-only base image on a separate qcow file by using

copy on write Copy-on-write (COW), sometimes referred to as implicit sharing or shadowing, is a resource-management technique used in computer programming to efficiently implement a "duplicate" or "copy" operation on modifiable resources. If a resource is dupl ...

. This new qcow file contains the path to the base image to be able to refer back to it when required. When a particular piece of data has to be read from this new image, the content is retrieved from it if it is new and was stored there; if it is not, the data is fetched from the base image. Optional features include

zlib zlib ( or "zeta-lib", ) is a software library used for data compression. zlib was written by Jean-loup Gailly and Mark Adler and is an abstraction of the DEFLATE compression algorithm used in their gzip file compression program. zlib is also a ...

-based transparent decompression.

qcow2

qcow2 is an updated version of the qcow format. qcow2 supports AES encryption. The difference from the original version is that qcow2 supports multiple snapshots using a newer, more flexible model for storing them. The official documentation for the format is part of the QEMU Git tree.

References

{{Reflist Disk images Filename extensions