Btrfs is a general purpose copy-on-write filesystem with extents,
writable snapshotting, support for multiple devices and many more
features focused on fault tolerance, repair and easy administration.

The filesystem disk format is no longer unstable, and it's not
expected to change unless there are strong reasons to do so. If there
is a format change, file systems with a unchanged format will
continue to be mountable and usable by newer kernels.

For more information, please see the web pages at
https://btrfs.readthedocs.io

To compile this file system support as a module, choose M here. The
module will be called btrfs.

If unsure, say N.

source code:

fs/btrfs/