248 lines
5.9 KiB
HTML
248 lines
5.9 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>/initrd</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Linux Filesystem Hierarchy"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Linux Filesystem Hierarchy"
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HREF="c23.html"><LINK
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REL="PREVIOUS"
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TITLE="/home"
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HREF="home.html"><LINK
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REL="NEXT"
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TITLE="/lib"
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HREF="lib.html"></HEAD
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><BODY
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CLASS="sect1"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Linux Filesystem Hierarchy: </TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="home.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 1. Linux Filesystem Hierarchy</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="lib.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="initrd"
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></A
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>1.8. /initrd</H1
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><P
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> <TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> initrd provides the capability to load a RAM disk by the boot
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loader. This RAM disk can then be mounted as the root file
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system and programs can be run from it. Afterwards, a new
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root file system can be mounted from a different device. The
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previous root (from initrd) is then moved to a directory and
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can be subsequently unmounted.
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initrd is mainly designed to allow system startup to occur
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in two phases, where the kernel comes up with a minimum set
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of compiled-in drivers, and where additional modules are
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loaded from initrd.
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Operation
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---------
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When using initrd, the system typically boots as follows:
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1) the boot loader loads the kernel and the initial RAM disk
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2) the kernel converts initrd into a "normal" RAM disk and
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frees the memory used by initrd
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3) initrd is mounted read-write as root
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4) /linuxrc is executed (this can be any valid executable,
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including shell scripts; it is run with uid 0 and can do
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basically everything init can do)
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5) linuxrc mounts the "real" root file system
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6) linuxrc places the root file system at the root directory
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using the pivot_root system call
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7) the usual boot sequence (e.g. invocation of /sbin/init) is
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performed on the root file system
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8) the initrd file system is removed
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Note that changing the root directory does not involve unmounting
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it. It is therefore possible to leave processes running on initrd
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during that procedure. Also note that file systems mounted under
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initrd continue to be accessible.
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Usage scenarios
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---------------
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The main motivation for implementing initrd was to allow
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for modular kernel configuration at system installation.
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The procedure would work as follows:
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1) system boots from floppy or other media with a minimal kernel
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(e.g. support for RAM disks, initrd, a.out, and the Ext2 FS)
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and loads initrd
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2) /linuxrc determines what is needed to (1) mount the "real" root
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FS (i.e. device type, device drivers, file system) and (2) the
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distribution media (e.g. CD-ROM, network, tape, ...). This can
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be done by asking the user, by auto-probing, or by using a
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hybrid approach.
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3) /linuxrc loads the necessary kernel modules
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4) /linuxrc creates and populates the root file system (this
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doesn't have to be a very usable system yet)
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5) /linuxrc invokes pivot_root to change the root file system and
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execs - via chroot - a program that continues the installation
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6) the boot loader is installed
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7) the boot loader is configured to load an initrd with the set of
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modules that was used to bring up the system (e.g. /initrd can
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be modified, then unmounted, and finally, the image is written
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from /dev/ram0 or /dev/rd/0 to a file)
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8) now the system is bootable and additional installation tasks
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can be performed
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The key role of initrd here is to re-use the configuration data
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during normal system operation without requiring the use of a
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bloated "generic" kernel or re-compiling or re-linking the kernel.
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A second scenario is for installations where Linux runs on systems
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with different hardware configurations in a single administrative
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domain. In such cases, it is desirable to generate only a small set
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of kernels (ideally only one) and to keep the system-specific part
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of configuration information as small as possible. In this case, a
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common initrd could be generated with all the necessary modules.
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Then, only /linuxrc or a file read by it would have to be different.
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A third scenario are more convenient recovery disks, because
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information like the location of the root FS partition doesn't have
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to be provided at boot time, but the system loaded from initrd can
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invoke a user-friendly dialog and it can also perform some sanity
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checks (or even some form of auto-detection).
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Last not least, CD-ROM distributors may use it for better installation
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from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
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via initrd from CD; or by booting via a loader like LOADLIN or directly
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from the CD-ROM, and loading the RAM disk from CD without need of floppies.
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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>
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</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="home.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="lib.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
|
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VALIGN="top"
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>/home</TD
|
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="c23.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
|
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VALIGN="top"
|
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>/lib</TD
|
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></TR
|
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></TABLE
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></DIV
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></BODY
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></HTML
|
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> |