On 26/01/2012 13:30, Todd Goodman wrote:
> * Joakim Tjernlund<joakim.tjernlund@...>  [120126 05:38]:
> [..]
>> When you update the RO layer you will be back to a single copy, the previous
>> release which is only in the overlay is gone, right?
> That's up to you.  You can manage your updates however you want.  For
> example, you can have your RO layer loop mounted out of a release image
> and have two (or more) different RW areas depending on which image
> you're running.  That can allow you to fallback from an update fairly
> easily.

Indeed.

So for example I am trying something a bit like the Slax distro:
- I produce a base distro that will hopefully change very slowly. This 
has the minimal software for any other base (uclibc, busybox, few tools ...)
- I produce a faster moving update that has real software on it (eg I 
need Dovecot/Postfix, some other stuff)
- I have a final overlay with "my" software on, ie integration stuff 
(this is the web apps, very specific stuff to this build, etc.  note 
this may turn out to be a wrong move - considering combining this with 
the step above...)
- I bind mount some other stuff over the top of this, eg my kernel 
/lib/modules is in a squashfs and bind mounted into this.  Therefore I 
can update the kernel separately to all of the above.


The goal of this chain was to minimise the *size* of updates.  Your goal 
might be different and hence have fewer/more layers.  I also considered 
the possibility of "shims" between the faster moving layers above.  The 
motivation was to be able to do little micro updates without rebuilding 
a whole layer (my layers are perhaps 10MB+ and the users have expensive 
and slow bandwidth, so being able to ship a 50KB shim to fix something 
is better than a 10MB module - not convinced yet, but sharing the idea...)

Don't feel this is right for you.  Just highlighting how you could use 
an overlay filesystem.  Also see the Slax distribution for an example 
which takes this to extremes (software installs are just overlay modules 
to a basic core system - very clever)

Good luck

Ed W