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swift 12/04/05 19:40:55 |
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|
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Modified: metadoc.xml |
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Added: initramfs-guide.xml |
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Log: |
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Fix bug #408971 - Add an initramfs-guide |
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|
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Revision Changes Path |
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1.238 xml/htdocs/doc/en/metadoc.xml |
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|
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file : http://sources.gentoo.org/viewvc.cgi/gentoo/xml/htdocs/doc/en/metadoc.xml?rev=1.238&view=markup |
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plain: http://sources.gentoo.org/viewvc.cgi/gentoo/xml/htdocs/doc/en/metadoc.xml?rev=1.238&content-type=text/plain |
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diff : http://sources.gentoo.org/viewvc.cgi/gentoo/xml/htdocs/doc/en/metadoc.xml?r1=1.237&r2=1.238 |
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|
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Index: metadoc.xml |
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=================================================================== |
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RCS file: /var/cvsroot/gentoo/xml/htdocs/doc/en/metadoc.xml,v |
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retrieving revision 1.237 |
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retrieving revision 1.238 |
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diff -u -r1.237 -r1.238 |
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--- metadoc.xml 15 Jan 2012 15:54:44 -0000 1.237 |
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+++ metadoc.xml 5 Apr 2012 19:40:55 -0000 1.238 |
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@@ -1,8 +1,8 @@ |
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<?xml version="1.0" encoding="UTF-8"?> |
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<!DOCTYPE metadoc SYSTEM "/dtd/metadoc.dtd"> |
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-<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/metadoc.xml,v 1.237 2012/01/15 15:54:44 swift Exp $ --> |
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+<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/metadoc.xml,v 1.238 2012/04/05 19:40:55 swift Exp $ --> |
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<metadoc lang="en"> |
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- <version>1.159</version> |
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+ <version>1.160</version> |
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<members> |
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<lead>nightmorph</lead> |
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<member>cam</member> |
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@@ -357,6 +357,7 @@ |
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<file id="conky">/doc/en/conky-howto.xml</file> |
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<file id="man-guide">/doc/en/man-guide.xml</file> |
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<file id="info-guide">/doc/en/info-guide.xml</file> |
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+ <file id="initramfs-guide">/doc/en/initramfs-guide.xml</file> |
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<file id="java-upgrade">/proj/en/java/java-upgrade.xml</file> |
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<file id="kernel-config">/doc/en/kernel-config.xml</file> |
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<file id="zsh">/doc/en/zsh.xml</file> |
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@@ -1093,6 +1094,9 @@ |
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<doc fileid="info-guide"> |
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<memberof>sysadmin_general</memberof> |
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</doc> |
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+ <doc fileid="initramfs-guide"> |
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+ <memberof>sysadmin_general</memberof> |
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+ </doc> |
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<doc fileid="java-upgrade"> |
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<memberof>upgrade</memberof> |
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</doc> |
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|
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|
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|
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1.1 xml/htdocs/doc/en/initramfs-guide.xml |
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|
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file : http://sources.gentoo.org/viewvc.cgi/gentoo/xml/htdocs/doc/en/initramfs-guide.xml?rev=1.1&view=markup |
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plain: http://sources.gentoo.org/viewvc.cgi/gentoo/xml/htdocs/doc/en/initramfs-guide.xml?rev=1.1&content-type=text/plain |
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|
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Index: initramfs-guide.xml |
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=================================================================== |
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<?xml version='1.0' encoding="UTF-8"?> |
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<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/initramfs-guide.xml,v 1.1 2012/04/05 19:40:55 swift Exp $ --> |
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|
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<!DOCTYPE guide SYSTEM "/dtd/guide.dtd"> |
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|
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<guide> |
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<title>Gentoo Linux Initial RAM File System Guide</title> |
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|
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<author title="Author"> |
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<mail link="swift@g.o">Sven Vermeulen</mail> |
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</author> |
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|
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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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|
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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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|
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<version>1</version> |
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<date>2012-03-31</date> |
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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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</body> |
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</section> |
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</chapter> |
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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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: |
| 374 |
</p> |
| 375 |
|
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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 |
| 378 |
-dmsquash-live -gensplash -iscsi -livenet -mdraid -multipath -nbd -nfs -plymouth |
| 379 |
-ssh-client" |
| 380 |
</pre> |
| 381 |
|
| 382 |
<p> |
| 383 |
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> |
| 386 |
|
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<p> |
| 388 |
The next step is to configure <c>dracut</c> by editing |
| 389 |
<path>/etc/dracut.conf</path>. In the configuration file, which is well |
| 390 |
commented, you can add in support for specific modules where needed. |
| 391 |
</p> |
| 392 |
|
| 393 |
<p> |
| 394 |
Once configured, create an initramfs by calling <c>dracut</c> as follows: |
| 395 |
</p> |
| 396 |
|
| 397 |
<pre caption="Calling dracut to generate an initramfs"> |
| 398 |
# <i>dracut</i> |
| 399 |
</pre> |
| 400 |
|
| 401 |
<p> |
| 402 |
The resulting image supports generic system boots based on the configuration in |
| 403 |
<path>/etc/dracut.conf</path>. You can also opt to generate an initramfs |
| 404 |
specifically tailored to your system (in which <c>dracut</c> tries to detect the |
| 405 |
needed tools, drivers, etc. from your existing system). If you know that the |
| 406 |
needed support (code and drivers) is built in in your kernel (and not as |
| 407 |
module), then you can even add in <c>--no-kernel</c>: |
| 408 |
</p> |
| 409 |
|
| 410 |
<pre caption="Allowing dracut to generate a host-specific initramfs"> |
| 411 |
# <i>dracut --host-only --no-kernel</i> |
| 412 |
</pre> |
| 413 |
|
| 414 |
<p> |
| 415 |
For more information, check out the <c>dracut</c> and <c>dracut.cmdline</c> |
| 416 |
manual pages. |
| 417 |
</p> |
| 418 |
|
| 419 |
</body> |
| 420 |
</section> |
| 421 |
</chapter> |
| 422 |
|
| 423 |
<chapter> |
| 424 |
<title>Additional resources</title> |
| 425 |
<section> |
| 426 |
<title>Gentoo-specific</title> |
| 427 |
<body> |
| 428 |
|
| 429 |
<ul> |
| 430 |
<li> |
| 431 |
<uri link="http://en.gentoo-wiki.com/wiki/Initramfs">Initramfs</uri> on |
| 432 |
Gentoo-wiki.com |
| 433 |
</li> |
| 434 |
<li> |
| 435 |
<uri link="http://wiki.gentoo.org/wiki/Initramfs">Initramfs</uri> on the |
| 436 |
official Gentoo Wiki |
| 437 |
</li> |
| 438 |
<li> |
| 439 |
<uri link="http://wiki.gentoo.org/wiki/Dracut">Dracut</uri> on the official |
| 440 |
Gentoo Wiki |
| 441 |
</li> |
| 442 |
</ul> |
| 443 |
|
| 444 |
</body> |
| 445 |
</section> |
| 446 |
<section> |
| 447 |
<title>General resources</title> |
| 448 |
<body> |
| 449 |
|
| 450 |
<ul> |
| 451 |
<li> |
| 452 |
<uri |
| 453 |
link="https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt">ramfs-rootfs-initramfs.txt</uri> |
| 454 |
within the Linux kernel documentation |
| 455 |
</li> |
| 456 |
</ul> |
| 457 |
|
| 458 |
</body> |
| 459 |
</section> |
| 460 |
</chapter> |
| 461 |
|
| 462 |
</guide> |
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