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<br />
<h1><big>larch</big>
–
a do-it-yourself live <em>Arch Linux</em> CD</h1>
<img style="border: 0px solid ; width: 320px; height: 320px;" alt="" src="larch1.jpg"
name="graphics1" align="right" hspace="10" vspace="10" />
<br /><br />
<h2>Structure of a <em>larch</em> live CD</h2>
<p>The vast majority of the code that runs in a live CD/USB system is exactly
the same as in a normal system running from hard disk.
The main differences arise through the live CD/USB system being based on
a read-only root file-system, so various adjustments are necessary to
make the areas that must be writable appear to be so. I say 'appear to be'
because in general any changes will be lost when the system shuts down,
the actual changes being made only in a RAM-based filesystem (but see
<a href="larch_sessionsave.html">"Session saving"</a>).
</p>
<h3><a name="Squashfs_and_Unionfs"></a>squashfs and aufs</h3>
<p>In <em>larch</em>, as in many other live CD/USB systems, the ability to (apparently)
write to a read-only file-system is provided by <em>aufs</em> (originally
derived from <em>unionfs</em>).
The file-system is also compressed, using <em>squashfs</em>, so as to fit more data
onto the storage device (normally about 2GB of uncompressed code and data
can be squeezed onto a 700MB CDROM using this approach). A custom
<em>Arch Linux</em> installation (you can choose which packages are installed) is
compressed into a <em>squashfs</em> file-system in the file
<strong>system.sqf</strong>, which is placed in the root directory of the boot device.
</p>
<p>For use in a live CD/USB system a few changes must be made to some files in the
installation. In <em>larch</em> this is (at first) not done directly, the installed
system is left in a 'clean' state. Instead of that, an additional <em>union</em> layer
is created, in the archive <strong>mods.sqf</strong>,
containing all the modified files (except those in /etc, which are placed in the
top, writable, <em>union</em> layer, which is saved in the <strong>overlay.ovl</strong> archive.
The user is free to include (pretty well) any customizations (s)he wants in these
overlay files. These files are also placed in the root directory of the boot device.
</p>
<p>These system archives are combined when the live CD/USB system
boots. They are mounted as branches of a <em>union</em> file-system with
the overlay above the 'standard' system, so that files in the overlay have
priority over the original ones. The top layer of the <em>union</em> is a <em>tmpfs</em>
writable layer (thus, effectively, making the whole system writable),
into which the archive <strong>overlay.ovl</strong> is unpacked during booting.
As the changes are only stored in RAM, they will, however, be lost when the system shuts
down (unless using the <a href="larch_sessionsave.html">session saving</a> feature).
</p>
<h3><a name="initramfs"></a><em>initramfs</em> and <em>mkinitcpio</em></h3>
<p>It is, in general, not possible to boot directly into a live CD/USB system.
Some form of 'initrd' or 'initramfs' is required, in order to find the boot
device, get hold of the file-system and prepare it for running.
Many modern <em>GNU/Linux</em> systems use initrd/initramfs (the former being now deprecated)
as a matter of course, for loading the necessary kernel modules, and
<em>Arch Linux</em> is no exception, a modular initramfs system (<em>mkinitcpio</em>)
being the standard way to boot <em>Arch Linux</em>, and <em>larch</em> also takes
advantage of it, although some additions need to be made to boot live CD/USB systems.
</p>
<h3><a name="rc_files"></a>/etc/inittab, /etc/rc.sysinit and /etc/rc.shutdown</h3>
<p>The initial boot script and the shutdown script must be customized for use
in a larch live environment. There are numerous ways of achieving that and
<em>larch</em> has tried out several. In version 5.3.2 a method was introduced which
involves replacing the original /etc/rc.sysinit and /etc/rc.shutdown scripts
by modified versions which are fully equivalent to the official scripts but
split off certain parts into functions which can be overridden when used in a
<em>larch</em> live system. The 'larch-live' package provides the files
/etc/larch-sysinit and /etc/larch-shutdown which provide the new versions
of these functions for live use.
<p>The need to overwrite certain system files is a bit messy, and means that
the initscripts package should not be updated in the live system, but I
hope the official package will sometime itself contain suitable versions of the
'/etc/rc.sysinit' and '/etc/rc.shutdown' files which separate out the
relevant functions so that larch doesn't need to overwrite them.
</p>
<p>Although it is not strictly necessary (because the substituted files are
fully equivalent to the official ones), larch saves the originals as
'/etc/rc.sysinit.larchsave' and '/etc/rc.shutdown.larchsave', so that these
can be restored on installation by <em>larchin</em> (this might avoid later
confusion).
</p>
<p>It is possible to use a customized version of '/etc/inittab', by
putting the new version in the <em>profile</em> (in 'rootoverlay') - for
example one could customize the handling of the first terminal, e.g. to
allow automatic login.
The original <em>inittab</em> is saved as '/etc/inittab.larchsave',
so that it can be restored if the system is installed using <em>larchin</em>.
</p>
<p>One addition in the <em>larch</em> version of the initscripts is the
automatic generation of a new '/etc/fstab' (using the script
<strong>gen_fstab</strong>, which
bases the result on the devices which are detected in the system). Otherwise
it behaves much the same as the version in
a normal <em>Arch Linux</em> system. However, the remount of the root-filessystem and
file-system checks have been left out as they are not relevant in a live-CD
environment. No partitions are mounted and, by default, swap is not enabled.
The reason is primarily that for use as an installation CD, it is better not
to have any partitions mounted, because it is quite likely that the partitions
will be edited in preparation for the installation. Passing boot parameter
'swap' will enable swap if there is a suitable partition (or it can be done
manually using 'swapon -a').
</p>
<p>'/etc/rc.shutdown' is adapted by
adding code to deal with session-saving (writing changes back to the boot medium)
and to eject the live-CD. Unlike in the normal version, the hardware clock is not reset.
</p>
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