https://lwn.net/Articles/332802/
https://pkalever.wordpress.com/2016/01/22/xfs-reflinks-tutorial/
https://lwn.net/Articles/331808/
https://blogs.oracle.com/devpartner/entry/whats_new_in_oracle_linux
https://blogs.oracle.com/wim/entry/ocfs2_reflink
https://lkml.org/lkml/2016/10/12/176