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<h1><span><em>live Arch Linux</em> builder</span></h1>
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<li><a href="larch_sessionsave.html"><h6>Next:</h6>
<div class="indent1">Saving a session back to the boot device (or even somewhere else)</div></a></li>
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<h2 id="pagetitle" level="1">Structure of a <em>larch live</em> system</h2>
<p>The vast majority of the code that runs in a <em>live</em> CD/USB system
is exactly the same as in a normal system running from hard disk.
The main differences arise through the <em>live</em> system being based on
a read-only root file-system, so various adjustments are necessary to
make the areas that must be writeable 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 <em>live</em> 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. 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 'larch' directory of the boot device.
</p>
<p>For use in a <em>live</em> system a few changes must be made to some files
in the installation. In <em>larch</em> this is usually 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. The user is free to include (pretty well)
any customizations (s)he wants in this overlay file, which is also placed
in the 'larch' directory of the boot device.
</p>
<p>Using <em>larch</em>'s merge-overlay feature it is possible to
rebuild the 'mods.sqf' archive to incorporate subsequent changes to the
system. When this is done an additional <em>union</em> layer
(<strong>filter.sqf</strong>) will be created. For further details see the
<a href="larch_sessionsave.html">session saving page</a>.
</p>
<p>These system archives are combined when the <em>live</em> system
boots. They are mounted as branches of a <em>union</em> (<em>aufs</em>)
file-system with the overlay above the 'standard' system, so that files in
the overlay have priority over the original ones. A further, <em>tmpfs</em>,
layer is placed on top of these, which effectively makes the whole system
writeable. When <a href="larch_sessionsave.html">session saving</a> is being
used this top layer is initialized during the boot sequence, by copying the
contents of a compressed archive (<strong>overlay.tar.lzo</strong>) into it.
<em>lzo</em> compression is used here because of its speed ('lzop' is a
dependency of the 'larch-live' package).
</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 <em>live</em>
system.
Some form of 'initrd' or 'initramfs' is required, in order to find the boot
device, prepare the <em>union</em> 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 <em>live</em> systems.
</p>
<p><em>larch</em> adds several 'hooks' (code plug-ins) to the
<em>Arch Linux mkinitcpio</em> system (they are in the 'larch-live' package).
The first, 'larch1', is concerned with finding the boot device.
It can select a device on the basis
of UUID, partition label, partition name ('/dev/sdb1', etc.) or else it can
search all available devices for the file '/larch/larchboot'. A simple
validity check is made by testing the existence of the '/larch/system.sqf'
archive.
</p>
<p>The second hook comes in two flavours - one for <em>aufs</em> and one
for <em>unionfs</em> - it just sets some variables according to which
unioning file-system is being used (at the moment only <em>aufs</em>
is properly supported).
</p>
<p>The third hook, 'larch3', performs the actual preparation of the
<em>larch</em> root file-system, combining the system and overlay archives
with a <em>tmpfs</em> upper layer in a <em>union</em> and carrying out
various other initializations.
</p>
<p>In order to generate the special <em>initramfs</em> a customized
'/etc/mkinitcpio.conf' is used, prepared during the build process from the
template '/etc/mkinitcpio.conf.larch0' and saved as
'/etc/mkinitcpio.conf.larch'. If you want to customize this, you need to
edit '/etc/mkinitcpio.conf.larch0' in 'rootoverlay' in the profile (the
gui has a button for this).
</p>
<p>There is also a customized <em>mkinitcpio</em> preset in '/etc/mkinitcpio.d',
also working with a template - 'larch.preset0' is used to produce 'larch.preset'
during the build operation. In addition this preset replaces the standard
preset belonging to the kernel, so that subsequent kernel updates (etc.)
will regenerate a correct <em>initramfs</em> for the <em>larch</em> system.
If you want to revert to the normal <em>Arch Linux mkinitcpio</em> behaviour
(for example, if you are installing the system) you will need to replace
the preset by the original, which gets saved with '.larchsave' appended.
</p>
<h3><a name="rc_files"></a>/etc/rc.sysinit and /etc/rc.shutdown</h3>
<p>The initial boot script and the shutdown script must be customized for use
in a <em>larch live</em> environment. This is achieved by using hooks in the
standard /etc/rc.sysinit and /etc/rc.shutdown scripts. The file
'etc/rc.d/functions.d/larch-hooks', in the 'larch-live' package, defines these.
During startup the main tweaks are to the content of /etc/mtab and /etc/fstab.
During shutdown we probably want to disable writing to the hardware clock, but
the main addition is the session saving code. If booting from CD/DVD we will
want to eject this just before shutting down.
</p>
<h3><a name="fstab"></a>/etc/fstab</h3>
<p>If the file '/etc/fstab.larch' exists this will be used as a persistent
'/etc/fstab', being copied there during the boot process (in the 'larch3'
hook in the initramfs system). Otherwise a very minimal '/etc/fstab' will
be generated at each boot. It includes entries for any existing swap
partitions, but these are commented out unless the 'swap' boot parameter
is specified (the default <em>larch</em> behaviour is to mount nothing
automatically).
</p>
<p>The 'x10d_fstab' script in the 'larch-live' package allows the very basic
/etc/fstab generated by the initramfs system ('larch3' hook) to be extended
automatically, but '/etc/fstab' will only be changed if '/etc/fstab.larch'
doesn't exist.
This script adds entries for the partitions it detects, and also mount points
in '/mnt' for them. The entries are such that the partitions are not mounted
automatically.
</p>
<h3><a name="aufs"></a>[<em>unionfs as an alternative to aufs</em>]</h3>
<p>In principle, <em>unionfs</em> can be used instead of <em>aufs</em>,
but as <em>aufs</em> is supported by current <em>Arch Linux</em> kernels
it is obviously the first choice. Indeed the current <em>larch</em> code
probably won't work with <em>unionfs</em>, as it hasn't been tested - but
the framework to support it is in place.
</p>
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