man4/initrd.4

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.\" $Id: initrd.4,v 0.9 1997/11/07 05:05:32 kallal Exp kallal $
.TH INITRD 4 2010-09-04 "Linux" "Linux Programmer's Manual"
.SH NAME
initrd \- boot loader initialized RAM disk
.SH CONFIGURATION
The
.I /dev/initrd
is a read-only block device assigned
major number 1 and minor number 250.
Typically
.I /dev/initrd
is owned by
.I root.disk
with mode 0400 (read access by root only).
If the Linux system does not have
.I /dev/initrd
already created, it can be created with the following commands:
.nf
\fB
        mknod \-m 400 /dev/initrd b 1 250
        chown root:disk /dev/initrd
\fP
.fi
.PP
Also, support for both "RAM disk" and "Initial RAM disk"
(e.g.,
.BR CONFIG_BLK_DEV_RAM=y
and
.BR CONFIG_BLK_DEV_INITRD=y )
must be compiled directly into the Linux kernel to use
.IR /dev/initrd .
When using
.IR /dev/initrd ,
the RAM disk driver cannot be loaded as a module.
.\"
.\"
.\"
.SH DESCRIPTION
The special file
.I /dev/initrd
is a read-only block device.
This device is a RAM disk that is initialized (e.g., loaded)
by the boot loader before the kernel is started.
The kernel then can use
.IR /dev/initrd "'s "
contents for a two-phase system boot-up.
.PP
In the first boot-up phase, the kernel starts up
and mounts an initial root file-system from the contents of
.I /dev/initrd
(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.
.\"
.\"
.\"
.SS "Boot-up Operation"
When booting up with
.BR initrd ,
the system boots as follows:
.IP 1. 3
The boot loader loads the kernel program and
.IR /dev/initrd 's
contents into memory.
.IP 2.
On kernel startup,
the kernel uncompresses and copies the contents of the device
.I /dev/initrd
onto device
.I /dev/ram0
and then frees the memory used by
.IR /dev/initrd .
.IP 3.
The kernel then read-write mounts the device
.I /dev/ram0
as the initial root file system.
.IP 4.
If the indicated normal root file system is also the initial
root file-system (e.g.,
.IR /dev/ram0 )
then the kernel skips to the last step for the usual boot sequence.
.IP 5.
If the executable file
.IR /linuxrc
is present in the initial root file-system,
.I /linuxrc
is executed with UID 0.
(The file
.I /linuxrc
must have executable permission.
The file
.I /linuxrc
can be any valid executable, including a shell script.)
.IP 6.
If
.I /linuxrc
is not executed or when
.I /linuxrc
terminates, the normal root file system is mounted.
(If
.I /linuxrc
exits with any file-systems mounted on the initial root
file-system, then the behavior of the kernel is
.BR UNSPECIFIED .
See the NOTES section for the current kernel behavior.)
.IP 7.
If the normal root file system has a directory
.IR /initrd ,
the device
.I /dev/ram0
is moved from
.IR /
to
.IR /initrd .
Otherwise if the directory
.IR /initrd
does not exist, the device
.I /dev/ram0
is unmounted.
(When moved from
.IR /
to
.IR /initrd ,
.I /dev/ram0
is not unmounted and therefore processes can remain running from
.IR /dev/ram0 .
If directory
.I /initrd
does not exist on the normal root file system
and any processes remain running from
.IR /dev/ram0
when
.I /linuxrc
exits, the behavior of the kernel is
.BR UNSPECIFIED .
See the NOTES section for the current kernel behavior.)
.IP 8.
The usual boot sequence (e.g., invocation of
.IR /sbin/init )
is performed on the normal root file system.
.\"
.\"
.\"
.SS Options
The following boot loader options, when used with
.BR initrd ,
affect the kernel's boot-up operation:
.TP
.BI initrd= "filename"
Specifies the file to load as the contents of
.IR /dev/initrd .
For
.B LOADLIN
this is a command-line option.
For
.B LILO
you have to use this command in the
.B LILO
configuration file
.IR /etc/lilo.config .
The filename specified with this
option will typically be a gzipped file-system image.
.TP
.I noinitrd
This boot option disables the two-phase boot-up operation.
The kernel performs the usual boot sequence as if
.I /dev/initrd
was not initialized.
With this option, any contents of
.I /dev/initrd
loaded into memory by the boot loader contents are preserved.
This option permits the contents of
.I /dev/initrd
to be any data and need not be limited to a file system image.
However, device
.I /dev/initrd
is read-only and can be read only one time after system startup.
.TP
.BI root= "device-name"
Specifies the device to be used as the normal root file system.
For
.B LOADLIN
this is a command-line option.
For
.B LILO
this is a boot time option or
can be used as an option line in the
.B LILO
configuration file
.IR /etc/lilo.config .
The device specified by the this option must be a mountable
device having a suitable root file-system.
.\"
.\"
.\"
.SS "Changing the Normal Root File System"
By default,
the kernel's settings
(e.g., set in the kernel file with
.BR rdev (8)
or compiled into the kernel file),
or the boot loader option setting
is used for the normal root file systems.
For an NFS-mounted normal root file system, one has to use the
.B nfs_root_name
and
.B nfs_root_addrs
boot options to give the NFS settings.
For more information on NFS-mounted root see the kernel documentation file
.BR Documentation/filesystems/nfsroot.txt .
For more information on setting the root file system see also the
.BR LILO
and
.BR LOADLIN
documentation.
.PP
It is also possible for the
.I /linuxrc
executable to change the normal root device.
For
.I /linuxrc
to change the normal root device,
.IR /proc
must be mounted.
After mounting
.IR /proc ,
.I /linuxrc
changes the normal root device by writing into the proc files
.IR /proc/sys/kernel/real-root-dev ,
.IR /proc/sys/kernel/nfs-root-name ,
and
.IR /proc/sys/kernel/nfs-root-addrs .
For a physical root device, the root device is changed by having
.I /linuxrc
write the new root file system device number into
.IR /proc/sys/kernel/real-root-dev .
For an NFS root file system, the root device is changed by having
.I /linuxrc
write the NFS setting into files
.IR /proc/sys/kernel/nfs-root-name
and
.I /proc/sys/kernel/nfs-root-addrs
and then writing 0xff (e.g., the pseudo-NFS-device number) into file
.IR /proc/sys/kernel/real-root-dev .
For example, the following shell command line would change
the normal root device to
.IR /dev/hdb1 :
.nf

    echo 0x365 >/proc/sys/kernel/real-root-dev

.fi
For an NFS example, the following shell command lines would change the
normal root device to the NFS directory
.I /var/nfsroot
on a local networked NFS server with IP number 193.8.232.7 for a system with
IP number 193.8.232.2 and named "idefix":
.nf

    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
.fi

.BR Note :
The use of
.I /proc/sys/kernel/real-root-dev
to change the root file system is obsolete.
See the Linux kernel source file
.I Documentation/initrd.txt
as well as
.BR pivot_root (2)
and
.BR pivot_root (8)
for information on the modern method of changing the root file system.
.\" FIXME the manual page should describe the pivot_root mechanism.
.\"
.\"
.\"
.SS Usage
The main motivation for implementing
.B initrd
was to allow for modular kernel configuration at system installation.
.PP
A possible system installation scenario is as follows:
.IP 1. 3
The loader program boots from floppy or other media with a minimal kernel
(e.g., support for
.IR /dev/ram ,
.IR /dev/initrd ,
and the ext2 file-system) and loads
.IR /dev/initrd
with a gzipped version of the initial file-system.
.IP 2.
The executable
.I /linuxrc
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.
.IP 3.
The executable
.I /linuxrc
loads the necessary modules from the initial root file-system.
.IP 4.
The executable
.I /linuxrc
creates and populates the root file system.
(At this stage the normal root file system does not have to be a
completed system yet.)
.IP 5.
The executable
.IR /linuxrc
sets
.IR /proc/sys/kernel/real-root-dev ,
unmount
.IR /proc ,
the normal root file system and any other file
systems it has mounted, and then terminates.
.IP 6.
The kernel then mounts the normal root file system.
.IP 7.
Now that the file system is accessible and intact,
the boot loader can be installed.
.IP 8.
The boot loader is configured to load into
.I /dev/initrd
a file system with the set of modules that was used to bring up the system.
(e.g., Device
.I /dev/ram0
can be modified, then unmounted, and finally, the image is written from
.I /dev/ram0
to a file.)
.IP 9.
The system is now bootable and additional installation tasks can be
performed.
.PP
The key role of
.I /dev/initrd
in the above is to reuse the configuration data during normal system operation
without requiring initial kernel selection, a large generic kernel or,
recompiling the kernel.
.PP
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
.I /linuxrc
file or a file executed by
.I /linuxrc
would be different.
.PP
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
.I /dev/initrd
can use a dialog and/or auto-detection followed by a
possible sanity check.
.PP
Last but not least, Linux distributions on CD-ROM may use
.B initrd
for easy installation from the CD-ROM.
The distribution can use
.B LOADLIN
to directly load
.I /dev/initrd
from CD-ROM without the need of any floppies.
The distribution could also use a
.B LILO
boot floppy and then bootstrap a bigger RAM disk via
.IR /dev/initrd
from the CD-ROM.
.\"
.\"
.\"
.SH FILES
.I /dev/initrd
.br
.I /dev/ram0
.br
.I /linuxrc
.br
.I /initrd
.\"
.\"
.\"
.SH NOTES
.IP 1. 3
With the current kernel, any file systems that remain mounted when
.I /dev/ram0
is moved from
.I /
to
.I /initrd
continue to be accessible.
However, the
.I /proc/mounts
entries are not updated.
.IP 2.
With the current kernel, if directory
.I /initrd
does not exist, then
.I /dev/ram0
will
.B not
be fully unmounted if
.I /dev/ram0
is used by any process or has any file-system mounted on it.
If
.IR /dev/ram0
is
.B not
fully unmounted, then
.I /dev/ram0
will remain in memory.
.IP 3.
Users of
.I /dev/initrd
should not depend on the behavior give in the above notes.
The behavior may change in future versions of the Linux kernel.
.\"
.\"
.\"
.\" .SH AUTHORS
.\" The kernel code for device
.\" .BR initrd
.\" was written by Werner Almesberger <almesber@lrc.epfl.ch> and
.\" Hans Lermen <lermen@elserv.ffm.fgan.de>.
.\" The code for
.\" .BR initrd
.\" was added to the baseline Linux kernel in development version 1.3.73.
.SH "SEE ALSO"
.BR chown (1),
.BR mknod (1),
.BR ram (4),
.BR freeramdisk (8),
.BR rdev (8)

.I Documentation/initrd.txt
in the Linux kernel source tree, the LILO documentation,
the LOADLIN documentation, the SYSLINUX documentation