1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<html>
<head>
<meta http-equiv="Content-type" content="text/html; charset=utf-8" />
<title>Structure of a larch live system</title>
<!-- (en) Add your meta data here -->
<link href="css/larchdocs.css" rel="stylesheet" type="text/css"/>
<!--[if lte IE 7]>
<link href="css/yaml/core/iehacks.css" rel="stylesheet" type="text/css" />
<![endif]-->
</head>
<body>
<!-- skip link navigation -->
<ul id="skiplinks">
<li><a class="skip" href="#col1">Skip to main content (Press Enter).</a></li>
</ul>
<div class="page_margins">
<div class="page">
<div id="top"><div id="tm"></div></div>
<!-- begin: #col1 -->
<div id="col1" role="main">
<div id="col1_content">
<div id="header" role="banner">
<h1><span><em>live Arch Linux</em> builder</span></h1>
</div>
<!-- begin: #col3 navigation column -->
<div id="col3" role="complementary">
<div id="col3_content">
<img class="indent2" alt="larch logo" src="css/screen/larch150x.png" width="150" height="150" />
<div class="vlist">
<ul>
<li><a href="index.html"><h6>Table Of Contents</h6></a></li>
<li><a href="larch_ssh.html"><h6>Next:</h6>
<div class="indent1">ssh access</div></a></li>
<li><a href="profiles.html"><h6>Previous:</h6>
<div class="indent1">Profiles</div></a></li>
</ul>
</div>
</div>
</div>
<!-- end: #col3 -->
<div class="larchdocs">
<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> 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 the bulk of the underlying system is generally (as is the case in a
<em>larch</em> system) stored in a <em>squashfs</em> archive, which is not
writeable. This is overcome to some extent by using <em>aufs</em> to add
a writeable overlay.
</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>aufs</em> layer is created containing all the modified files (in a writeable
system also changes made by the user will be placed in this layer). You can
include (pretty well) any customisations you like in this overlay, giving you
a great deal of freedom in designing your <em>live</em> system.
</p>
<p>When the <em>live</em> system boots an <em>aufs</em> file-system is set up
with the base system as its lowest layer (branch). The modification layer - either
the 'mods.sqf' file or the medium itself (if writeable) - is
placed above this. If the modification layer is not writeable (either inherently
or because of a boot option) a further, writeable, layer in <em>tmpfs</em>
(a memory based file-system) is added above this. The <em>aufs</em> file-system
is then used as the root file-system for the <em>live</em> system (compare with
a normal linux system where the root file-system would be a hard-disk partition).
</p>
<h3><a name="bundles"></a><em>'Bundles'</em></h3>
<p>An additional possibility inherent in <em>aufs</em> is that of including
additional code or data 'packages' - which I here call 'bundles', to distinguish
them from the normal packages managed by pacman - by adding them as further
layers (branches). This is also supported in <em>larch</em>. It is a completely
optional feature, supported in the <em>initramfs</em> code, by the 'larch' hook
(see the man-pages or the <em>Arch wiki</em> for more details on
<em>mkinitcpio</em> and <em>Arch initramfs</em> support). Bundles are simply
overlays over the root file system (like software packages) which should
be compressed into <em>squashfs</em> archives with names ending in '.sqb'.
They should be placed in the medium's '/larch/bundles' directory and during
the boot process they will be added to the <em>aufs</em> file-system in
alphabetical order directly above the base system layer.
</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 and prepare the root file-system 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 its own 'hook' (code plug-in) to the
<em>Arch Linux mkinitcpio</em> system (the 'larch' hook is supplied in the
'larch-live' package). It performs several steps, the first of which is to
find the live medium.
</p>
<p>Note that the live medium is not necessarily the same as the
boot device (which could be a boot-cd required for booting from another
device unsupported by the machine's BIOS). The medium can be detected / selected
on the basis of a UUID, a partition label, or a partition device node
('/dev/sdb1', etc.). Alternatively, all available devices can be searched
for the file '/larch/larchboot'. Once the device has been found a simple
validity check is made by testing for the existence of '/larch/system.sqf'.
</p>
<p>An <em>aufs</em> file-system is set up and the <em>squashfs</em> archive
'system.sqf' is added as the base layer. If there are 'bundles' in the
'/larch/bundles' directory these are layered on top of the base (as
<em>aufs</em> branches).
</p>
<p>Then the modifications overlay is added (as a further <em>aufs</em>
branch), and if the overlay is read-only then another layer is added on
top of it, a writeable in-memory file-system (<em>tmpfs</em>).
</p>
<p>A few more tweaks are made to prepare the file-system for the boot
proper (see the code of the larch hook in '/lib/initcpio/hooks/larch'
for the details) and this <em>aufs</em> file-system is then used as the
root file-system for the further boot process, after leaving the
<em>initramfs</em>.
</p>
<p>In order to generate the special <em>initramfs</em> a customised
'/etc/mkinitcpio.conf' - '/etc/mkinitcpio.conf.larch' - is used, also
supplied in the 'larch-live' package (though it can be replaced by a
customised version in the profile's overlay directory - the
gui has a button for this).
</p>
<p>There is also a customized <em>mkinitcpio</em> preset,
'/etc/mkinitcpio.d/larch.preset', which is used to replace 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,
and if running from CD/DVD we will want to eject the device 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 'larch'
hook in the <em>initramfs</em> 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 <em>initramfs</em> system ('larch' 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>
</div>
<div class="topref"><a href="#top">Top</a></div>
</div>
<!-- begin: #footer -->
<div id="footer">
<div id="footer-content" role="contentinfo">© 2010 Michael Towers<br />
Page layout assisted by <a href="http://www.yaml.de/">YAML</a> and
<a href="http://www.kuwata-lab.com/tenjin/">pyTenjin</a>
</div>
</div>
<!-- end: #footer -->
</div>
<!-- end: #col1 -->
<div id="bottom"><div id="bl"><div id="bm"></div></div></div>
</div>
</div>
<!-- full skiplink functionality in webkit browsers -->
<script src="css/yaml/core/js/webkit-focusfix.js" type="text/javascript"></script>
</body>
</html>
|
