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<HTML><HEAD><TITLE>Linux Programmer's Manual: INITRD (4)</TITLE>
</HEAD><body bgcolor=#ffffff text=#000000 link=#4f4fff vlink=#0000ff alink=#00ff00>
This is the initrd manual page converted to HTML.
<p>
<hr>
<A NAME="lbAB"><H2>NAME</H2></A>
initrd - boot loader initialized RAM disk
<A NAME="lbAC"> </A>
<H2>DESCRIPTION</H2>
The special file
<B>/dev/initrd </B>
is a read-only block device.
Device
<B>/dev/initrd</B>
is a RAM disk that is initialized (e.g. loaded) by the boot loader before
the kernel is started.
The kernel then can use the the block device
<B>/dev/initrd</B>'s
contents for a two phased system boot-up.
<P>
In the first boot-up phase, the kernel starts up
and mounts an initial root file-system from the contents
of
<B>/dev/initrd </B>
(e.g. RAM disk initialized by the boot loader).
In the second phase, additional drivers or other modules
are loaded from the initial root device's contents.
After loading the additional modules, a new root file system
(i.e. the normal root file system) is mounted from a
different device.
<A NAME="lbAD"> </A>
<H2>BOOT-UP OPERATION </H2>
When booting up with
<B>initrd</B>, the system boots as follows:
<DL COMPACT><DT><DD>
<P>
1. The boot loader loads the kernel program and
<B>/dev/initrd</B>'s contents into memory.
<P>
2. On kernel startup, the kernel uncompresses and copies the contents of the device
<B>/dev/initrd</B>
onto device
<B>/dev/ram0 </B>
and then frees the memory used by
<B>/dev/initrd</B>.
<P>
3. The kernel then read-write mounts device
<B>/dev/ram0 </B>
as the initial root file system.
<P>
4. If the indicated normal root file system is also the initial root file-system
(e.g.
<B>/dev/ram0</B>
) then the kernel skips to the last step for the usual boot sequence.
<P>
5. If the executable file
<B>/linuxrc</B> is present in the initial root file-system, <B>/linuxrc</B>
is executed with uid 0.
(The file
<B>/linuxrc</B>
must have executable permission.
The file
<B>/linuxrc</B>
can be any valid executable, including a shell script.)
<P>
6. If
<B>/linuxrc</B>
is not executed or when
<B>/linuxrc</B>
terminates, the normal root file system is mounted.
(If
<B>/linuxrc</B>
exits with any file-systems mounted on the initial root
file-system, then the behavior of the kernel is
<B>UNSPECIFIED</B>.
See the
<B>NOTES</B>
section for the current kernel behavior.)
<P>
7. If the normal root file has directory
<B>/initrd</B>, device
<B>/dev/ram0</B>
is moved from
<B>/</B> to <B>/initrd</B>.
Otherwise if directory
<B>/initrd</B> does not exist device <B>/dev/ram0</B> is unmounted.
(When moved from
<B>/</B> to <B>/initrd</B>, <B>/dev/ram0</B>
is not unmounted and therefore processes can remain running from
<B>/dev/ram0</B>.
If directory
<B>/initrd</B>
does not exist on the normal root file-system
and any processes remain running from
<B>/dev/ram0</B> when <B>/linuxrc</B>
exits, the behavior of the kernel is
<B>UNSPECIFIED</B>.
See the
<B>NOTES</B>
section for the current kernel behavior.)
<P>
8. The usual boot sequence (e.g. invocation of
<B>/sbin/init</B>) is performed on the normal root file system.
</DL>
<A NAME="lbAE"> </A>
<H2>OPTIONS</H2>
The following boot loader options when used with
<B>initrd</B>, affect the kernel's boot-up operation:
<DL COMPACT>
<DT><B>initrd=</B><I>filename</I>
<DD>
Specifies the file to load as the contents of
<B>/dev/initrd</B>.
For <B>LOADLIN</B>
this is a command line option.
For <B>LILO</B> you have to use this command in the
<B>LILO</B> configuration file <B>/etc/lilo.config</B>.
The filename specified with this
option will typically be a gzipped file-system image.
<DT><B>noinitrd</B>
<DD>
This boot time option disables the two phase boot-up operation.
The kernel performs the usual boot sequence as if
<B>/dev/initrd </B>
was not initialized.
With this option, any contents of
<B>/dev/initrd </B>
loaded into memory by the boot loader contents are preserved.
This option permits the contents of
<B>/dev/initrd</B>
to be any data and need not be limited to a file system image.
However, device
<B>/dev/initrd </B>
is read-only and can be read only one time after system startup.
<DT><B>root=</B><I>device-name</I>
<DD>
Specifies the device to be used as the normal root file system.
For <B>LOADLIN</B>
this is a command line option.
For <B>LILO</B> this is a boot time option or
can be used as an option line in the
<B>LILO</B> configuration file <B>/etc/lilo.config</B>.
The device specified by the this option must be a mountable
device having a suitable root file-system.
</DL>
<A NAME="lbAF"> </A>
<H2>CHANGING THE NORMAL ROOT FILE SYSTEM</H2>
By default,
the kernel's settings
(i.e. set in the kernel file with
<B>rdev</B> or compiled into the kernel file ),
or the boot loader option setting
is used for the normal root file systems.
For a NFS-mounted normal root file system, one has to use the
<B>nfs_root_name</B> and <B>nfs_root_addrs</B>
boot options to give the NFS settings.
For more information on NSF-mounted root see the kernel documentation file
<B>nfsroot.txt</B>.
For more information on setting the root file system also see the
<B>LILO</B> and <B>LOADLIN</B> documentation.
<P>
It is also possible for the
<B>/linuxrc</B>
executable to change the normal root device.
For
<B>/linuxrc</B>
to change the normal root device,
<B>/proc</B> must be mounted.
After mounting
<B>/proc</B>, <B>/linuxrc</B>
changes the normal root device by writing into the proc files
<B>/proc/sys/kernel/real-root-dev</B>,
<B>/proc/sys/kernel/nfs-root-name</B>, and
<B>/proc/sys/kernel/nfs-root-addrs</B>.
For a physical root device, the root device is changed by having
<B>/linuxrc</B>
write the new root file system device number into
<B>/proc/sys/kernel/real-root-dev</B>.
For a NSF root file system, the root device is changed by having
<B>/linuxrc</B>
write the NSF setting into files
<B>/proc/sys/kernel/nfs-root-name</B> and
<B>/proc/sys/kernel/nfs-root-addrs</B>
and then writing 0xff (e.g. the pseudo-NFS-device number) into file
<B>/proc/sys/kernel/real-root-dev</B>.
For example, the following shell command line would change the normal root device to
<B>/dev/hdb1 :</B>
<PRE>
<B>
echo 0x365 >/proc/sys/kernel
</B>
</PRE>
For a NSF example, the following shell command lines would change the
normal root device to the NSF directory
<B>/var/nfsroot</B>
on a local networked NSF server with IP number 193.8.232.7 for a system with
IP number 193.8.232.7 and named 'idefix':
<PRE>
<B>
echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
echo 193.8.232.2:193.8.232.7::255.255.255.0:idefix \
>/proc/sys/kernel/nfs-root-addrs
echo 255 >/proc/sys/kernel/real-root-dev
</B>
</PRE>
<A NAME="lbAG"> </A>
<H2>USAGE</H2>
The main motivation for implementing
<B>initrd</B>
was to allow for modular kernel configuration at system installation.
<P>
A possible system installation scenario is as follows:
<DL COMPACT><DT><DD>
<P>
1. The loader program boots from floppy or other media with a minimal kernel
(e.g. support for
<B>/dev/ram</B>, <B>/dev/initrd</B>, and the ext2 file-system) and loads
<B>/dev/initrd</B> with a gzipped version of the initial file-system.
<P>
2. The executable
<B>/linuxrc</B>
determines what is needed to (1) mount the normal root file-system
(i.e. device type, device drivers, file system) and (2) the
distribution media (e.g. CD-ROM, network, tape, ...). This can be
done by asking the user, by auto-probing, or by using a hybrid
approach.
<P>
3. The executable
<B>/linuxrc</B>
loads the necessary modules from the initial root file-system.
<P>
4. The executable
<B>/linuxrc</B>
creates and populates the root file system. (At this stage the normal
root file system does not have to be a
completed system yet.)
<P>
5. The executable
<B>/linuxrc</B> sets <B>/proc/sys/kernel/real-root-dev,</B>
unmount
<B>/proc</B>,
the normal root file system and any other file
systems it has mounted, and then terminates.
<P>
6. The kernel then mounts the normal root file system.
<P>
7. Now that the file system is accessible and intact,
the boot loader can be installed.
<P>
8. The boot loader is configured to load into
<B>/dev/initrd</B>
a file system with the set of modules that was used to bring up the system.
(e.g. Device
<B>/dev/ram0</B>
can be modified, then unmounted, and finally, the image is written from
<B>/dev/ram0</B>
to a file.)
<P>
9. The system is now bootable and additional installation tasks can be
performed.
</DL>
<P>
The key role of
<B>/dev/initrd</B>
in the above is to re-use the configuration data during normal system operation
without requiring initial kernel selection, a large generic kernel or,
recompiling the kernel.
<P>
A second scenario is for installations where Linux runs on systems with
different hardware configurations in a single administrative network.
In such cases, it may be desirable to use only a small set of kernels
(ideally only one) and to keep the system-specific part of configuration
information as small as possible.
In this case, create a common file
with all needed modules.
Then, only the the
<B>/linuxrc</B>
file or a file executed by
<B>/linuxrc</B>
would be different.
<P>
A third scenario is more convenient recovery disks.
Because information like the location of the root file-system
partition is not needed at boot time, the system loaded from
<B>/dev/initrd </B>
can use a dialog and/or auto-detection followed by a
possible sanity check.
<P>
Last but not least, Linux distributions on CD-ROM may use
<B>initrd</B>
for easy installation from the CD-ROM.
The distribution can use
<B>LOADLIN</B>
to directly load
<B>/dev/initrd</B>
from CD-ROM without the need of any floppies.
The distribution could also use a
<B>LILO</B>
boot floppy and then bootstrap a bigger ram disk via
<B>/dev/initrd</B> from the CD-ROM.
<A NAME="lbAH"> </A>
<H2>CONFIGURATION</H2>
The
<B>/dev/initrd </B>
is a read-only block device assigned
major number 1 and minor number 250.
Typically
<B>/dev/initrd</B>
is owned by
<B>root.disk </B>
with mode 0400 (read access by root only).
If the Linux system does not have
<B>/dev/initrd</B>
already created, it can be created with the following commands:
<PRE>
<B>
mknod -m 400 /dev/initrd b 1 250
chown root.disk /dev/initrd
</B>
</PRE>
Also, support for both "RAM disk" and "Initial RAM disk"
(e.g.
<B>CONFIG_BLK_DEV_RAM=y</B> and <B>CONFIG_BLK_DEV_INITRD=y</B>
) support must be compiled directly into the Linux kernel to use
<B>/dev/initrd</B>.
When using
<B>/dev/initrd</B>,
the RAM disk driver cannot be loaded as a module.
<A NAME="lbAI"> </A>
<H2>FILES</H2>
<I>/dev/initrd</I>
<BR>
<I>/dev/ram0</I>
<BR>
<I>/linuxrc</I>
<BR>
<I>/initrd</I>
<A NAME="lbAJ"> </A>
<H2>SEE ALSO</H2>
<B><A HREF="http:/cgi-bin/man2html?man1/chown.1">chown</A></B>(1), <B><A HREF="http:/cgi-bin/man2html?man1/mknod.1">mknod</A></B>(1), <B>/dev/<A HREF="http:/cgi-bin/man2html?man4/ram.4">ram</A></B>(4),
<B><A HREF="http:/cgi-bin/man2html?man8/freeramdisk.8">freeramdisk</A></B>(8), <B><A HREF="http:/cgi-bin/man2html?man8/rdev.8">rdev</A></B>(8),
<BR>
documentation file <B>initrd.txt</B> in the kernel source package,
<BR>
<B>LILO</B> documentation, <B>LOADLIN</B> documentation,
<B>SYSLINUX</B> documentation.
<A NAME="lbAK"> </A>
<H2>NOTES</H2>
1. With the current kernel, any file systems that remain mounted when
<B>/dev/ram0</B> is moved from <B>/</B> to <B>/initrd</B>
continue to be accessible. However, the
<B>/proc/mounts</B>
entries are not updated.
<P>
2. With the current kernel, if directory
<B>/initrd</B> does not exist, then <B>/dev/ram0</B>
will NOT be fully unmounted if
<B>/dev/ram0</B>
is used by any process or has any file-system mounted on it.
If
<B>/dev/ram0</B> is NOT fully unmounted,
then
<B>/dev/ram0</B>
will remain in memory.
<P>
3. Users of
<B>/dev/initrd</B>
should not depend on the behavior give in the above notes.
The behavior may change in future versions of the Linux kernel.
<A NAME="lbAL"> </A>
<H2>AUTHOR</H2>
The kernel code for device
<B>initrd</B>
was written by Werner Almesberger <<A HREF="mailto:almesber@lrc.epfl.ch">almesber@lrc.epfl.ch</A>> and
Hans Lermen <<A HREF="mailto:lermen@elserv.ffm.fgan.de">lermen@elserv.ffm.fgan.de</A>>.
The code for
<B>initrd</B>
was added to the baseline Linux kernel in development version 1.3.73
<HR>
This manual page was converted to HTML using
<A HREF="http:/cgi-bin/man2html">man2html</A>,
18:06:48 EET, December 13, 1997
</BODY>
</HTML>
|
