Bug#558688: live-helper: support hybrid iso/usb-hdd images

[intrigeri@boum.org]

Package: live-helper
Version: 2.0~a1-1
Severity: wishlist

Hi,

as Daniel, being the syslinux maintainer in Debian, already knows,
syslinux >=3.72 supports some kind of hybrid images which can either
be burnt on CD-ROM or raw-copied to a disk (USB stick, etc.); this is
practically achieved by post-processing an ISO image using the bundled
isohybrid script.

Rationale
=========

It would be nice to have live-helper optionally do such
post-processing on the ISO images it builds. It would allow people who
ship live systems built with Debian Live to build, publish and
distribute half less images => less needed disk space, less bandwidth,
more convenient for users, etc.

Would you agree with this?

The other popular way to {build and ship, download} less images is to
use some ISO->USB solution, such as UNetbootin[1], but these tend to
replace our beloved bootloader, that was carefully prepared by Debian
Live, with their own one... potentially loosing most boot options that
were initially proposed to the Live system user. That's why I believe
the hybrid ISO/HDD way to be far better.

  [1] http://unetbootin.sourceforge.net/

Implementation
==============

I'm thinking of implementing this as an optional step done in the
binary.sh script created and run by lh_binary_iso.

As such tweaked ISO images still haven't been tested on a large enough
set of hardware, this would probably need to be a opt-in option.

As Lenny's syslinux does not support booting such images, I'm
pondering between different ways of managing this configuration
variable; my current plan is to always accept it at lh_config time,
but warn if building for <Squeeze, anderror out in lh_binary_iso if
the chroot does not support isohybrid. One could also want to error
out at lh_config time if building for <Squeeze, but it would forbid
building a Lenny image that has a backported, recent enough syslinux
package installed... which I intend to do actually.

Any thoughts?

Details
=======

Please note I had to use the "-entry 4 -type 1c" isohybrid options to
have the resulting ISO boot on my test system once dd'ed to a USB
stick. Being quite ignorant in the field of MBR & partition tables,
here are the explanations I was able to find on the Internet:
- using 4 as the partition number is supposed to help with BIOSes
  that only support USB-Zip boot
- using 1c (i.e. hidden FAT32 LBA), instead of the default 0x17
  (hidden NTFS, IIRC), as the partition type is sometimes needed to
  get the BIOS even look at the partition created by isohybrid.

Unless a "perfect" options combination is found, I'm in favor of
making this configurable.

Bye,
--
  intrigeri <intrigeri@boum.org>
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