overlayfs-tools

OverlayFS is the union filesystem provided by Linux kernel.

This program comes provides three tools:

vacuum - remove duplicated files in upperdir where copy_up is done but the file is not actually modified (see the sentence "the copy_up may turn out to be unnecessary" in the Linux documentation). This may reduce the size of upperdir without changing lowerdir or overlay.
diff - show the list of actually changed files (the difference between overlay and lowerdir). A file with its type changed (i.e. from symbolic link to regular file) will shown as deleted then added, rather than modified. Similarly, for a opaque directory in upperdir, the corresponding directory in lowerdir (if exists) will be shown as entirely deleted, and a new directory with the same name added. File permission/owner changes will be simply shown as modified.
merge - merge down the changes from upperdir to lowerdir. Unlike aubrsync for AuFS which bypasses the union filesystem mechanism, overlayfs-utils emulates the OverlayFS logic, which will be far more efficient. After this operation, upperdir will be empty and lowerdir will be the same as original overlay.
deref - copy changes from upperdir to uppernew while unfolding redirect directories and metacopy regular files, so that new upperdir is compatible with legacy overlayfs driver.

For safety reasons, vacuum and merge will not actually modify the filesystem, but generate a shell script to do the changes instead.

Prerequisite and Building

To build it, you must have libattr headers installed. The package is usually named libattr1-dev or libattr-devel. For example,

# apt-get install libattr1-dev

Then, you just build this program:

$ make

# ./overlay diff -l /lower -u /upper

See ./overlay --help for more.

A directory is made opaque by setting the xattr "trusted.overlay.opaque" to "y".

However, only users with CAP_SYS_ADMIN can read trusted.* extended attributes.

Only works for regular files and directories. Do not use it on OverlayFS with device files, socket files, etc..
Hard links may be broken (i.e. resulting in duplicated independent files).
File owner, group and permission bits will be preserved. File timestamps, attributes and extended attributes might be lost.
This program only works for OverlayFS with only one lower layer.
It is recommended to have the OverlayFS unmounted before running this program.