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 btrfs.

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. Being able to revert to previous file versions is like gold dust on traditional file servers. However, if you use btrfs to host very large data files, such as virtual disk files, the CoW technology can perform slow writes.

Using the chattr command...