HOW TO HANDLE ISO RDIFFS on DEVELOPER
		   ----------------------------------------

Author: Finn-Arne Johansen faj@bzz.no

This Document is released under the GNU General Public License, version 2.
On Debian GNU/Linux systems, the complete text of the GNU General Public
License can be found in `/usr/share/common-licenses/GPL'.

$Id: handling_iso_rdiffs.txt,v 1.2 2005/07/22 15:05:24 pere Exp $

This HOWTO is based on the work in the test lab of BzzWare AS
http://bzz.no.

Background for Document
-----------------------

Developer.skolelinux.no is our developement machine. It's used for
various purposes, cvs repository, CD building, Some web applications
and for storing our CD-Images. but space is limitted, and diskspace
for a reliable server is costly. It's about backup, backup procedures,
reliablilty etc.

But at least we like to be able to regenerate the old Images we've
released, so we need to com up with a nice algorithm.Storing them as
full size, or even gzipped images takes a log of space.

That's why we use the same method as with slbackup, storing the rdiff
between each image. In addition, we keep some images fully, to make it
faster to regenerate the older images.

How to create an rdiff
----------------------

First you need to decide which image to keep, and which image to only
keep as an rdiff. 
Lets say you have 3 images:
 skolelinux-i386-rc1.iso
 skolelinux-i386-rc2.iso 
 skolelinux-i386-rc3.iso
We want to keep rc3, and only stor the rdiff's for rc1 and rc2. 

First we need to create the rdiff from rc2 to rc1. 
To create an rdiff, we first need to create a signature:
 rdiff signature skolelinux-i386-rc2.iso skolelinux-i386-rc2.sign

then we create the actual rdiff as a delta
 rdiff delta skolelinux-i386-rc2.sign skolelinux-i386-rc1.iso skolelinux-i386-rc1.rdiff

To make it easier to get the correct date of the image, set the rdiff
to hold the same date as the iso:
 touch -r skolelinux-i386-rc1.iso skolelinux-i386-rc1.rdiff

Then remove the iso for rc1, and the signature for rc2
 rm  skolelinux-i386-rc1.iso skolelinux-i386-rc2.sign

okay, some space was saved, but lets save mor, by creating an rdiff from
rc3 to rc2 as well:
 rdiff signature skolelinux-i386-rc3.iso skolelinux-i386-rc3.sign
 rdiff delta skolelinux-i386-rc3.sign skolelinux-i386-rc2.iso skolelinux-i386-rc2.rdiff
 touch -r skolelinux-i386-rc2.iso skolelinux-i386-rc2.rdiff
 rm  skolelinux-i386-rc2.iso skolelinux-i386-rc3.sign

This should leav us with 3 files:
 skolelinux-i386-rc1.rdiff
 skolelinux-i386-rc2.rdiff 
 skolelinux-i386-rc3.iso

How to Rebuild an iso image, using rdiffs
-----------------------------------------

Rebuilding the iso's is somewhat easier, but remember to use the
rdiff-patches the correct way.

We have the 2 given rdiff's and the one iso: 
 skolelinux-i386-rc1.rdiff
 skolelinux-i386-rc2.rdiff 
 skolelinux-i386-rc3.iso

We want to have an rc1.iso. 
First you must create an rc2.iso:
 rdiff patch skolelinux-i386-rc3.iso skolelinux-i386-rc2.rdiff skolelinux-i386-rc2.iso

then you must create the rc1.iso:
 rdiff patch skolelinux-i386-rc2.iso skolelinux-i386-rc1.rdiff skolelinux-i386-rc1.iso

Then set the correct date for the rc1.iso:
 touch -r skolelinux-i386-rc1.rdiff skolelinux-i386-rc1.iso

and remove the intermediate rc2.iso
 rm skolelinux-i386-rc2.iso


Todo
----
It would have been nice to have a web-page that could generate the old
iso's for us whenever we want to regenerate them.
It would also be nice if we could add more patch at once, to remove the
need for intermediate images.


That's al folks