[lug] /etc backup and unionfs(?)

Michael J. Hammel mjhammel at graphics-muse.org
Fri Jun 8 09:39:57 MDT 2007

On Fri, 2007-06-08 at 09:04 -0600, Bear Giles wrote:
> Live CDs are now able to run off a root image on the cd media.  It looks 
> like there's a unionfs that allows a ramdisk to overlay the read-only disk.
> That made me think -- what if the hard disk's / image is solely what's 
> in the distribution packages and is read-only, and you overlay it with a 
> read-write partition?  Wouldn't that mean that all configuration changes 
> are in that overlay partition and easily backed up and restored?  It 
> would also be easy to check for unwanted modifications, e.g., attempts 
> to install compromised binaries.

Not sure I'm following this.  You're overlaying the root partition,
which is mounted read-only, with a read-write partition so that you can
easily do backups of the overlay?  I'm not clear on what that's buying

However, a similar mechanism is already used by several live CDs in
order to save configuration data (I think SLAX does it, probably a few
others).  The idea is that the files that need to be modified at run
time are mounted on an overlay that maps to a USB stick.  If you're
lucky, the USB stick is also bootable (and thus carries the LiveCD
instead of on a CDROM).  This would be very similar to what you propose
in that, with a DVD or USB stick to boot from, you could easily carry
the root partition from a full distribution (compressed).  There
wouldn't be a "backup" per se, but rather a simple writing of the
configuration data to the USB stick (the one acting as your read-write
overlay space) at run time.  When you're done, yank the USB stick and go
on your merry way, modified configs in hand.

> (Okay, you would need to make a few changes in /etc, but only those 
> required to boot the system.  Maybe nothing more than setting up 
> /etc/fstab to load the overlay.)

Possibly, although the LiveCDs I've seen all do it from an rc script at
boot time from the initial ramdisk image.

> Two additional benefits: restoration would be trivial since the root 
> partition would depend on nothing but the standard packages, and you 
> could even boot from a live CD that's been modified to load the disk 
> overlay instead of a ramdisk.

The implementations I've seen actually load the ramdisk first and then
mount the modified stuff as the overlay, right in place, before starting
the rest of the system.  
Michael J. Hammel                                    Senior Software Engineer
mjhammel at graphics-muse.org                           http://graphics-muse.org
When C++ is your hammer, everything looks like a thumb.
              -- Steven M. Haflich

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