Normally, files exist in a filesystem to keep data contained within them
separated; seeing data
exchanged directly between files is often a sign of filesystem
corruption. There are, however, use cases where it is desirable to be able
to perform a controlled swap of data between a pair of files. Darrick Wong
has recently posted a
patch set implementing this feature for the XFS filesystem, but also
making it available in a general way.

Source: LWN.net – [$] Atomic extent swapping for XFS