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| Filename extensions | .qcow, .qcow2, .qcow3 |
|---|---|
| Internet media type | application/x-qemu-disk |
| Developed by | QEMU |
| Type of format | Disk image |
| Compression | Yes |
| Open format? | Yes |
| Free format? | Yes |
qcow is afile format fordisk image files used byQEMU, ahosted virtual machine monitor.[1] It stands for "QEMUCopy 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 guestoperating systems. Three versions of the format exist: qcow, qcow2 and qcow3[2] 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.[3] Thee2fsprogs command e2image also has support for generating qcow2 files to avoid the use of sparse file support.[4]
One of the main characteristics of qcowdisk images is that files with this format can grow as data is added. This allows for smaller file sizes thanraw disk images, which allocate the whole image space to a file, even if parts of it are empty. This is particularly useful forfile systems that do not supportsparse files, such asFAT32.[5]
The qcow format also allows storing changes made to a read-only base image on a separate qcow file by usingcopy on write. 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.[5]
Optional features includezlib-basedtransparent decompression.[6]
qcow2 is an updated version of the qcow format. qcow2 supportsAES encryption.[7] 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.[8]