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<?xml version='1.0' encoding="UTF-8"?>
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<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/alsa-guide.xml,v 1.85 2012/02/13 08:33:31 nightmorph Exp $ -->
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<!DOCTYPE guide SYSTEM "/dtd/guide.dtd">
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<guide>
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<title>Gentoo Linux Initial RAM File System Guide</title>
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<author title="Author">
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<mail link="swift@gentoo.org">Sven Vermeulen</mail>
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</author>
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<abstract>
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More and more systems require an initramfs to boot up properly. In this guide,
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we tackle the concepts of the initramfs as well as how to properly create and
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manage initramfs instances.
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</abstract>
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<!-- The content of this document is licensed under the CC-BY-SA license -->
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<!-- See http://creativecommons.org/licenses/by-sa/3.0 -->
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<license/>
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<version>1</version>
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<date>2012-03-31</date>
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<chapter>
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<title>Initramfs concepts</title>
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<section>
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<title>Introduction</title>
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<body>
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<p>
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For many users, an initramfs system is of no concern. Their system uses a simple
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partitioning schema with no exotic drivers or setups (like encrypted file
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systems), so the Linux kernel is well able to hand over control to the
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<c>init</c> binary on their system. But for many systems, an initramfs is almost
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mandatory.
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</p>
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<p>
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The key concept to understanding what an initramfs is (or is needed for) is to
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understand how the Linux boot process works, even in a high-level approach.
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</p>
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</body>
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</section>
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<section>
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<title>Linux boot process</title>
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<body>
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<p>
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Once the Linux kernel has control over the system (which it gets after being
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loaded by the boot loader), it prepares its memory structures and drivers as
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good as it can. It then hands over control to an application (usually
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<c>init</c>) whose task it is to further prepare the system and make sure that,
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at the end of the boot process, all necessary services are running and the user
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is able to log on. The <c>init</c> application does that by launching, amongst
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other services, the <c>udev</c> daemon who will further load up and prepare the
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system based on the detected devices. When <c>udev</c> is launched, all
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remaining file systems that have not been mounted are mounted, and the remainder
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of services is started.
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</p>
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<p>
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For systems where all necessary files and tools reside on the same file system,
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the <c>init</c> application can perfectly control the further boot process. But
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when multiple file systems are defined (or more exotic installations are done),
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this might become a bit more tricky:
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</p>
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<ul>
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<li>
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When the <path>/usr</path> partition is on a separate file system, tools and
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drivers that have files stored within <path>/usr</path> cannot be used
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unless <path>/usr</path> is available. If those tools are needed to make
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<path>/usr</path> available, then we cannot boot up the system.
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</li>
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<li>
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If the root file system is encrypted, then the Linux kernel will not be able
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to find the <c>init</c> application, resulting in an unbootable system.
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</li>
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</ul>
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<p>
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The solution for this problem has since long been to use an <e>initrd</e>
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(initial root device).
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</p>
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</body>
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</section>
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<section>
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<title>The initial root disk</title>
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<body>
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<p>
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The <e>initrd</e> is an in-memory disk structure (ramdisk) that
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contains the necessary tools and scripts to mount the needed file systems before
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control is handed over to the <c>init</c> application on the root file system.
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The Linux kernel triggers the setup script (usually called <c>linuxrc</c> but
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that is not mandatory) on this root disk, which prepares the system, switches to
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the real root file system and then calls <c>init</c>.
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</p>
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<p>
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Although the initrd method is all that is needed, it had a few drawbacks:
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</p>
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<ul>
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<li>
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It is a full-fledged block device, requiring the overhead of an entire
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file system on it, and has a fixed size. Choose an initrd that is too small,
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and you won't be able to fit in all needed scripts. Make it too big, and
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you're wasting memory.
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</li>
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<li>
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Because it is a real device, it also consumes cache memory in the Linux
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kernel and is prone to the memory- and file management methods in use (such
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as paging), making it even worse in memory consumption.
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</li>
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</ul>
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<p>
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To resolve these (for some perhaps hardly called) issues, the initramfs was
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created.
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</p>
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</body>
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</section>
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<section>
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<title>The initial ram file system</title>
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<body>
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<p>
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An <e>initramfs</e> is an initial ram file system based on <e>tmpfs</e>
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(a size-flexible, in-memory lightweight file system), which also didn't use a
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separate block device (so no caching was done and all overhead mentioned earlier
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disappears). Just like the initrd, it contains the tools and scripts needed to
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mount the file systems before the <c>init</c> binary on the real root file
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system is called. These tools can be decryption abstraction layers (for
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encrypted file systems), logical volume managers, software raid, bluetooth
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driver based file system loaders, etc.
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</p>
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<p>
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The content of the initramfs is made by creating a cpio archive. <c>cpio</c> is
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an old (but proven) file archiver solution (and its resulting archive files are
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called <e>cpio files</e>). You can definitely compare it to <c>tar</c>. The
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choice of <c>cpio</c> here was because it is easier to implement (code-wise) and
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supported (back then) device files (which <c>tar</c> couldn't).
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</p>
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<p>
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All files, tools, libraries, configuration settings (if applicable), etc. are
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put into the cpio archive. This archive is then compressed using the <c>gzip</c>
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utility and stored alongside the Linux kernel. The boot loader will then offer
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it to the Linux kernel at boot time so the kernel knows an initramfs is needed.
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</p>
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<p>
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Once detected, the Linux kernel will create a tmpfs file system, extract the
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contents of the archive on it, and then launches the <c>init</c> script located
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in the root of the tmpfs file system. This script will then mount the real root
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file system (after making sure it can mount it, for instance by loading
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additional modules, preparing an encryption abstraction layer, etc.) as well as
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vital other file systems (such as <path>/usr</path> and <path>/var</path>).
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</p>
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<p>
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Once the root file system and the other vital file systems are mounted, the
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<c>init</c> script from the initramfs will switch the root towards the real root
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file system and finally call the <c>/sbin/init</c> on that system to continue
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the boot process.
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</p>
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</body>
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</section>
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</chapter>
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<chapter>
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<title>Creating an initramfs</title>
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<section>
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<title>Introduction and bootloader configuration</title>
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<body>
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<p>
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To create an initramfs, it is important that you know what additional drivers,
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scripts and tools you need to boot your system. For instance, if you use LVM,
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then you will need to support LVM tools on the initramfs. Likewise, if you use
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software RAID, you need <c>mdadm</c>, etc.
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</p>
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<p>
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Some tools exist that help you create initramfs' (compressed cpio archives) for
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your system. But for those that want total control, you can easily create your
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own initramfs as well.
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</p>
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<p>
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Once created, you will need to adjust the bootloader configuration to tell it
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that an initramfs is to be used. For instance, if the initramfs file is stored
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as <path>/boot/initramfs-3.2.2-gentoo-r5</path>, then the configuration in
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<path>/boot/grub/grub.conf</path> could look like so:
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</p>
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<pre caption="Example entry in grub.conf for booting with an initramfs">
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title Gentoo Linux 3.2.2-r5
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root (hd0,0)
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kernel /boot/kernel-3.2.2-gentoo-r5
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initrd /boot/initramfs-3.2.2-gentoo-r5
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</pre>
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</body>
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</section>
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<section id="genkernel">
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<title>Using genkernel</title>
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<body>
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<p>
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Gentoo's kernel building utility, <c>genkernel</c>, can be used to generate an
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initramfs, even if you didn't use <c>genkernel</c> to configure and build your
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kernel.
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</p>
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<p>
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To use <c>genkernel</c> for generating an initramfs, it is recommended that you
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include all necessary drivers and code that is needed to mount your
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<path>/</path> and <path>/usr</path> file systems in the kernel (and not as
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modules). Then, call <c>genkernel</c> as follows:
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</p>
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<pre caption="Running genkernel to setup an initramfs">
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# <i>genkernel --install --no-ramdisk-modules initramfs</i>
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</pre>
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<p>
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Depending on your system, you might want to add one or more of the following
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options:
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</p>
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<table>
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<tr>
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<th>Option</th>
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<th>Description</th>
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</tr>
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<tr>
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<ti>--disklabel</ti>
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<ti>Add support for <c>LABEL=</c> settings in your <path>/etc/fstab</path></ti>
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</tr>
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<tr>
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<ti>--dmraid</ti>
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<ti>Add support for fake hardware RAID</ti>
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</tr>
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<tr>
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<ti>--firmware</ti>
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<ti>Add in firmware code found on the system</ti>
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</tr>
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<tr>
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<ti>--gpg</ti>
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<ti>Add in GnuPG support</ti>
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</tr>
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<tr>
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<ti>--iscsi</ti>
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<ti>Add support for iSCSI</ti>
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</tr>
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<tr>
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<ti>--luks</ti>
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<ti>Add support for luks encryption containers</ti>
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</tr>
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<tr>
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<ti>--lvm</ti>
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<ti>Add support for LVM</ti>
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</tr>
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<tr>
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<ti>--mdadm</ti>
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<ti>Add support for software RAID</ti>
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</tr>
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<tr>
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<ti>--multipath</ti>
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<ti>Add support for multiple I/O access towards a SAN</ti>
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</tr>
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<tr>
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<ti>--zfs</ti>
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<ti>Add support for ZFS</ti>
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</tr>
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</table>
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<p>
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When finished, the resulting initramfs file will be stored in your
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<path>/boot</path>.
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</p>
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</body>
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</section>
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<section id="dracut">
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<title>Using dracut</title>
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<body>
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<warn>
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At the time of writing, dracut is not marked stable yet, so you might need to
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unmask it before continuing.
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</warn>
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<p>
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The <c>dracut</c> utility is created for the sole purpose of managing initramfs
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files. It uses a highly modular approach on which support you want to include
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and which not.
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</p>
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<p>
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When you install <c>dracut</c>, you will need to take care to include support
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for the correct <c>DRACUT_MODULES</c>. This is a variable you can set in
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<path>/etc/make.conf</path> to include support for specific setups:
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</p>
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<pre caption="Preparing to install dracut">
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DRACUT_MODULES="dmraid lvm syslog -biosdevname -btrfs -caps -crypt -crypt-gpg
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-dmsquash-live -gensplash -iscsi -livenet -mdraid -multipath -nbd -nfs -plymouth
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-ssh-client"
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</pre>
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<p>
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It is adviseable to set (or unset) those modules you need (and don't need).
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Afterwards, <c>emerge dracut</c> to install the utility on your system.
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</p>
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<p>
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The next step is to configure <c>dracut</c> by editing
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<path>/etc/dracut.conf</path>. In the configuration file, which is well
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commented, you can add in support for specific modules where needed.
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</p>
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<p>
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Once configured, create an initramfs by calling <c>dracut</c> as follows:
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</p>
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| 336 |
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|
<pre caption="Calling dracut to generate an initramfs">
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| 337 |
|
|
# <i>dracut</i>
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| 338 |
|
|
</pre>
|
| 339 |
|
|
|
| 340 |
|
|
<p>
|
| 341 |
|
|
The resulting image supports generic system boots based on the configuration in
|
| 342 |
|
|
<path>/etc/dracut.conf</path>. You can also opt to generate an initramfs
|
| 343 |
|
|
specifically tailored to your system (in which <c>dracut</c> tries to detect the
|
| 344 |
|
|
needed tools, drivers, etc. from your existing system). If you know that the
|
| 345 |
|
|
needed support (code and drivers) is built in in your kernel (and not as
|
| 346 |
|
|
module), then you can even add in <c>--no-kernel</c>:
|
| 347 |
|
|
</p>
|
| 348 |
|
|
|
| 349 |
|
|
<pre caption="Allowing dracut to generate a host-specific initramfs">
|
| 350 |
|
|
# <i>dracut --host-only --no-kernel</i>
|
| 351 |
|
|
</pre>
|
| 352 |
|
|
|
| 353 |
|
|
<p>
|
| 354 |
|
|
For more information, check out the <c>dracut</c> and <c>dracut.cmdline</c>
|
| 355 |
|
|
manual pages.
|
| 356 |
|
|
</p>
|
| 357 |
|
|
|
| 358 |
|
|
</body>
|
| 359 |
|
|
</section>
|
| 360 |
|
|
</chapter>
|
| 361 |
|
|
|
| 362 |
|
|
<chapter>
|
| 363 |
|
|
<title>Additional resources</title>
|
| 364 |
|
|
<section>
|
| 365 |
|
|
<title>Gentoo-specific</title>
|
| 366 |
|
|
<body>
|
| 367 |
|
|
|
| 368 |
|
|
<ul>
|
| 369 |
|
|
<li>
|
| 370 |
|
|
<uri link="http://en.gentoo-wiki.com/wiki/Initramfs">Initramfs</uri> on
|
| 371 |
|
|
Gentoo-wiki.com
|
| 372 |
|
|
</li>
|
| 373 |
|
|
<li>
|
| 374 |
|
|
<uri link="http://wiki.gentoo.org/wiki/Initramfs">Initramfs</uri> on the
|
| 375 |
|
|
official Gentoo Wiki
|
| 376 |
|
|
</li>
|
| 377 |
|
|
<li>
|
| 378 |
|
|
<uri link="http://wiki.gentoo.org/wiki/Dracut">Dracut</uri> on the official
|
| 379 |
|
|
Gentoo Wiki
|
| 380 |
|
|
</li>
|
| 381 |
|
|
</ul>
|
| 382 |
|
|
|
| 383 |
|
|
</body>
|
| 384 |
|
|
</section>
|
| 385 |
|
|
<section>
|
| 386 |
|
|
<title>General resources</title>
|
| 387 |
|
|
<body>
|
| 388 |
|
|
|
| 389 |
|
|
<ul>
|
| 390 |
|
|
<li>
|
| 391 |
|
|
<uri
|
| 392 |
|
|
link="https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt">ramfs-rootfs-initramfs.txt</uri>
|
| 393 |
|
|
within the Linux kernel documentation
|
| 394 |
|
|
</li>
|
| 395 |
|
|
</ul>
|
| 396 |
|
|
|
| 397 |
|
|
</body>
|
| 398 |
|
|
</section>
|
| 399 |
|
|
</chapter>
|
| 400 |
|
|
|
| 401 |
|
|
</guide>
|
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