Btrfs is a new filesystem with extents, writable snapshotting,
support for multiple devices and many more features.

Btrfs is highly experimental, and THE DISK FORMAT IS NOT YET
FINALIZED. You should say N here unless you are interested in
testing Btrfs with non-critical data.

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/