COW File

What are COW files and how to open them

Are you having problems opening a COW file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your COW files.

What is a COW file?

A .COW file is a Copy On Write Disk Image file.

Copy On Write Disk Image files belong to a disk image format created by QEMU when using User Mode Linux. They make use of a base image (left unmodified) plus separate storage of modified data so that only changes consume space, while the base remains read-only. The strategy allows many images to share the same template and keeps each individual image smaller by writing only sectors that have changed.

A COW image is produced by the vlmkcow tool in QEMU's User Mode Linux setup. It is based on an original raw disk image which remains untouched. The COW file records the path and modification time of that original image. Such images support a backing file so that upon reads they consult the original for unmodified sectors, and writes go into the COW image. Snapshot-style behavior is possible: a COW image may be used in read-only mode with writes diverging, and changes can be merged back into the base image under controlled operations.

This format is kept mainly for compatibility with older QEMU versions. It is not supported on Windows builds of QEMU.

The image begins with the ASCII sequence "QED" followed by a zero byte, occupying the first four bytes at offset zero.

How to open COW files

Important: Different programs may use files with the COW file extension for different purposes, so unless you are sure which format your COW file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested four different COW openers which you will find listed below.

Last updated: September 2, 2026

Various apps that use files with this extension

These apps are known to open certain types of COW files. Remember, different programs may use COW files for different purposes, so you may need to try out a few of them to be able to open your specific file.

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about COW file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.