qcow
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| Internet media type | application/x-qemu-disk |
|---|---|
| Developed by | QEMU |
| Type of format | Disk image |
| Container for | File system |
qcow stands for "QEMU Copy On Write" and denotes a disk storage optimization strategy that delays allocation of storage until it is actually needed. QEMU is an emulator and virtual machine container, and it can use a variety of virtual disk images which are generally associated with specific guests operating systems.
Features
qcow images grow as data is added, and support AES encryption or transparent decompression.
One disadvantage of qcow images is that they cannot be mounted with offset, in the way that raw images can be mounted with mount /path/to/image.img /mount/path -o loop,offset=32256.
qcow2 also supports data compression using zlib.
Supported formats
The set of possible disk image formats have commonly accepted short names:
- vvfat: Virtual VFAT
- vpc: Windows Virtual PC
- bochs: Bochs file system
- dmg: Macintosh disk image
- cloop: Linux compressed loop
- vmdk: VMware virtual machine disk format
- qcow: QEMU Copy-on-write
- qcow2: QEMU Copy-on-write (new in version 0.9)
- raw: Raw file system (no special format)
- cow: User-mode Linux copy-on-write
qcow2
qcow2 is a newer version of the qcow format. QEMU can use a base image which is read-only, and store all writes to the qcow2 image. Among the QEMU supported formats, this is the most versatile format. Features include smaller images (useful if the filesystem does not support holes, for example on FAT32), optional AES encryption, zlib based compression and support of multiple VM snapshots. qemu and xen have retained the qcow format for backwards compatibility. Users can easily convert qcow disk images to the qcow2 format.
Application
One of the most important application of qcow2 is when there are various client virtualization requests on the host machine for a similar infrastructure. In such cases, the image of the operating system is stored - along with the changes made to each image independent of the other. Changes may include updates, software installation etc. This reduces the disk space used as most clients on large virtualization platforms (PaaS) usually demand infrastucture on similar grounds - popularly referred to as server templates.
External links
- "QEMU Emulator User Documentation". Wiki.qemu.org. Retrieved 2011-12-12.
- "The QCOW2 Image Format". People.gnome.org. 2008-09-11. Retrieved 2011-12-12.
- "Mounting KVM qcow2 qemu disk images « btm.geek". Blog.loftninjas.org. 2008-10-27. Retrieved 2011-12-12.