Multiple releases on a disk

["Michael S. Fischer" <michael@dynamine.net>]

Hi folks,

I'm working on a project with my employer whose product's software is
installed atop Debian/woody.

One of the requirements of this product is that users must be able to
downgrade to the previous installation if an upgrade causes them
problems.  I thought I could fulfill this request by making two separate
root partitions, originally populating them with the same packages;
then, if a user upgraded the packages on the current root partition and
subsequently found problems, she could revert by simply booting the
alternate root partition.

I originally thought I could solve this problem by creating a partition
table like this:

/
/altroot
/var

Clearly, however, that won't work, because Debian package state
information (among other things that are tightly bound to the state of the
rest of the filesystem) are stored in /var.

So, now I'm left with this:

/
/root
/altroot
/altvar

Which could simply be condensed to 

/
/altroot

The problem is that there is some state in /var (specifically, /var/log) which
arguably should be visible no matter which root partition is active.

I'd like to solicit input from the collective on this somewhat sticky
situation.  What in /var should be considered specific to a software
installation, and what should be considered sharable -- if anything at
all?  If there's any other input you consider relevant, I'd like to hear
from you.

Thanks,

-- 
Michael S. Fischer / michael at dynamine.net / +1 650-533-4684
Lead Hacketeer, Dynamine Consulting, Silicon Valley, CA