One of the underpinning technologies that helps with the success of the
btrfs filesystem is Copy-On-Write (CoW).
CoW is used in logical volume management filesystems, including ZFS used in Solaris (an Oracle product), Microsoft's Volume Shadow Copy, and
These CoW filesystems allow you to take instant snapshots or backups. This is due to the fact that as a file is written and a copy of it is made; hence, Copy-on-Write. As traditional filesystems implement this, the virtual disk technology can also implement this
CoW technology in
qcow2. In this way, any allocated disk space in the
qcow2 disk file is not used on the host until it's written to.
For generic filesystems, you will find the
CoW technology very useful. ...