linux: OverlayFS

OverlayFS offers a quick way to capture ALL file changes to a base directory via copy on write (COW) stored in an upper directory, presented transparently merged in a virtual directory:

sudo mount \
  -t overlay \
  overlay
  -o lowerdir={DIR1},upperdir={DIR2} \
  {DIR3}

where:

• lowerdir, dir1 = Readonly base directory
• upperdir , dir2 = Copy on write changes directory
  • (including markers for deleted files)
• dir3 = target virtual directory
  • reads all changes from upper merged over lower
  • writes all changes to upper ONLY