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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"
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<br /><br />
<h2>Saving a session back to the boot device (or even somewhere else)</h2>
<p>An easy way to create a customized live-USB-stick is to start from an
existing one, make whatever changes you like, and then save the result back to
the USB-stick.
</p>
<h3><a name="overlays"></a>Overlay archives, <strong>overlay.ovl</strong> and
<strong>mods.sqf</strong></h3>
<p>In <em>larch</em> the <em>overlay</em> file-system (in the
<em>compressed</em> file 'overlay.ovl') can be rebuilt
from the state of the live system when it shuts down.
This is handled primarily by the script <strong>/opt/larch-live/session-save/saveovl</strong>, which is
called from the customized shutdown function.
If the medium is writable and contains a file 'larch/save', the user is offered the possibility of
rewriting the overlay archive, thus saving the current state.
</p>
<p>The main overlay archive (<strong>overlay.ovl</strong>) is unpacked into memory
at boot and repacked when saving the session at shutdown. When this archive grows
too large it will reduce the amount of available memory, perhaps to an unacceptable extent.
It will also lengthen boot and shutdown times. To counter this, it is possible to merge the
overlay archive into the middle <em>union</em> layer in <strong>mods.sqf</strong> (which is
normally not copied to memory), thus restoring the rapid load and save times
for <strong>overlay.ovl</strong>, and maximizing the available memory. I call this process
'overlay merging', and it is offered as a shutdown option.
It takes longer to execute than a normal session save, but from larch version 5.2
a new <em>aufs</em> option allows the merging to be done quicker than it was
previously and without significant extra memory requirements. In this version
a separate <em>aufs</em> union of the two overlay layers is mounted in which
the whiteouts are visible. This union can then be squashed directly, without
requiring the new, merged overlay to be rebuilt in memory. As it is a feature
of <em>aufs</em>, overlay merging is no longer supported for <em>unionfs</em>
(note that <em>unionfs</em> is at present not actively supported in <em>larch</em>
anyway).
</p>
<h3><a name="var_files"></a>Special places in /var</h3>
<p>It was considered undesirable to include some things in the '/var' directory
in the session saves, primarily because of the space they occupy. Firstly,
the standard <em>pacman</em> package cache, '/var/cache/pacman/pkg' - if you
do want to save your cache, you should either handle this manually or set
up another location using 'CacheDir' in /etc/pacman.conf'. Also the log files
(in '/var/log') and '/var/tmp' (temporary files, like '/tmp') are not saved.
</p>
<h3><a name="upd_packages"></a>Updating packages</h3>
<p>It should be possible to include package addition and removal in the list
of customizations undertaken using this method, but there are some limitations. Apart from
size considerations (the top level overlay is unpacked into memory at boot), there is also
a limitation connected with the kernel. Essentially, the kernel and its modules may not
be updated. <strong>If you do this, you may well make a mess of your system</strong>.
In other words, be very careful with 'pacman -Su' - only upgrade selected packages.
</p>
<p>The reason for this is that the kernel is not contained within the
squashed file-system, but is held separately on the boot medium, while its modules
are contained in the squashed system and also in the initramfs. Updating these is not
impossible (if booting from a writable medium), but <em>pacman</em> cannot do it and
it is not straightforward (e.g. using a CD to boot to a USB-stick on a computer not
supporting USB boot would additionally need a new CD). A complete rebuild of the live
system might be a better approach. And using 'larchify' it should even be possible to
perform such a rebuild from within the running live system.
</p>
<h3><a name="de-Act"></a>Activating and deactivating the session-saving feature</h3>
<p>As mentioned above, for this feature to be available, there must be a file with
the name 'save' in the boot medium's 'larch' directory.
</p>
<h3><a name="custom"></a>Customizing the behaviour using scripts</h3>
<p>Some aspects of the session saving can be customized by writing scripts to
override the default behaviour. These scripts are (in general) kept on the boot
medium, in the 'larch' directory. As these are shell scripts and their use requires
some knowledge of the inner workings of the <em>larch</em> live system, it will
almost certainly be necessary to read the existing <em>larch</em> scripts where the
custom scripts are 'sourced' if you want to
write your own scripts to change the behaviour of the <em>larch</em> live system.
</p>
<ul>
<li><strong>session-save</strong>
<p>Used in <strong>/opt/larch-live/session-save/saveovl</strong>. This script can be
used to customize the creation of the new overlay archive, e.g. by
specifying a new location. The default is 'overlay.ovl' on the boot medium.
</p>
</li>
<li><strong>save-menu</strong>
<p>Used in <strong>/opt/larch-live/session-save/saveovl</strong>. This script can be
used to further customize the creation of the new overlay archive, e.g. by
providing an alternative session save menu. Note that if the
<strong>session-save</strong> script specified an alternative device path
for saving the overlay, this script will be sought in the 'larch' directory
on that path.
</p>
</li>
<li><strong>merge-adjust</strong>
<p>Used in <strong>/opt/larch-live/session-save/merge_overlay</strong>. This script can be
used to further customize aspects of the overlay merging process. Note that if the
<strong>session-save</strong> script specified an alternative device path
for saving the overlay, this script will be sought in the 'larch' directory
on that path.
</p>
</li>
<li><strong>boot-init</strong>
<p>In initramfs hook <strong>larch3</strong>. This script allows adjustment of
the overlay handling during the boot process, e.g. getting them from another
path.
Note that the shell environment here is that of the initramfs and thus
very limited.
</p>
</li>
</ul>
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