diff options
Diffstat (limited to 'build_tools/larch8/larch0/docs/html/larch_live_system.html')
| -rw-r--r-- | build_tools/larch8/larch0/docs/html/larch_live_system.html | 227 |
1 files changed, 227 insertions, 0 deletions
diff --git a/build_tools/larch8/larch0/docs/html/larch_live_system.html b/build_tools/larch8/larch0/docs/html/larch_live_system.html new file mode 100644 index 0000000..c827bdf --- /dev/null +++ b/build_tools/larch8/larch0/docs/html/larch_live_system.html @@ -0,0 +1,227 @@ +<!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> |