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diff --git a/build_tools/l7/larch0/docs/html/larch_live_system.html b/build_tools/l7/larch0/docs/html/larch_live_system.html new file mode 100644 index 0000000..1f7bbeb --- /dev/null +++ b/build_tools/l7/larch0/docs/html/larch_live_system.html @@ -0,0 +1,220 @@ +<!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_sessionsave.html"><h6>Next:</h6> + <div class="indent1">Saving a session back to the boot device (or even somewhere else)</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> 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> + + +</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> |