Cowloop

Cowloop (copy-on-write loop driver) was a Linux kernel driver that allows read-only filesystems to be made writable using the kernel module cowloop.ko. Cowloop worked by writing modified blocks to a separate file known as a cowfile, which is created by using the cowdev command to associate a block device, such as a CD, to a writable file.

Cowloop is now largely deprecated in favor of the device-mapper snapshot target which provides the same functionality and is also integrated with LVM2 (Linux Logical Volume Manager version 2).

Cowloop provided a somewhat similar functionality to unionfs and the newer aufs, but operates on a block level instead of operating on a file system level.

It is free software released under the terms of the GNU General Public License v2 or later.