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Show Whiteout Mode (shwh)
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Generally aufs hides the name of whiteouts. But in some cases, to show
them is very useful for users. For instance, creating a new middle layer
(branch) by merging existing layers.

(borrowing aufs1 HOW-TO from a user, Michael Towers)
When you have three branches,
- Bottom: 'system', squashfs (underlying base system), read-only
- Middle: 'mods', squashfs, read-only
- Top: 'overlay', ram (tmpfs), read-write

The top layer is loaded at boot time and saved at shutdown, to preserve
the changes made to the system during the session.
When larger changes have been made, or smaller changes have accumulated,
the size of the saved top layer data grows. At this point, it would be
nice to be able to merge the two overlay branches ('mods' and 'overlay')
and rewrite the 'mods' squashfs, clearing the top layer and thus
restoring save and load speed.

This merging is simplified by the use of another aufs mount, of just the
two overlay branches using the 'shwh' option.
# mount -t aufs -o ro,shwh,br:/livesys/overlay=ro+wh:/livesys/mods=rr+wh \
	aufs /livesys/merge_union

A merged view of these two branches is then available at
/livesys/merge_union, and the new feature is that the whiteouts are
visible!
Note that in 'shwh' mode the aufs mount must be 'ro', which will disable
writing to all branches. Also the default mode for all branches is 'ro'.
It is now possible to save the combined contents of the two overlay
branches to a new squashfs, e.g.:
# mksquashfs /livesys/merge_union /path/to/newmods.squash

This new squashfs archive can be stored on the boot device and the
initramfs will use it to replace the old one at the next boot.
