Fl-cow allows you to utilise hard links to save disk space by causing hard-linked files to be copied rather than overwritten as they are changed.

This is useful for making changes to large source trees while keeping a copy of the original tree to generate patches from. Also, if you are using the Arch revision control system with working copies hard-linked to a revision library, using fl-cow can prevent revision library corruption.

Fl-cow is an LD_PRELOAD hack, not a kernel patch.