Name
initrd — boot loader initialized RAM disk
DESCRIPTION
The special file /dev/initrd
is a read-only block device. Device /dev/initrd 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 block device
/dev/initrd's contents for a
two phased system boot-up.
In the first boot-up phase, the kernel starts up and
mounts an initial root file-system from the contents of
/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.
BOOT-UP OPERATION
When booting up with initrd, the system boots as
follows:
1. The boot loader loads the kernel program and
/dev/initrd's contents into
memory.
2. On kernel startup, the kernel uncompresses and
copies the contents of the device /dev/initrd onto device /dev/ram0 and then frees the memory
used by /dev/initrd.
3. The kernel then read-write mounts device
/dev/ram0 as the initial
root file system.
4. If the indicated normal root file system is also
the initial root file-system (e.g. /dev/ram0 ) then the kernel skips to
the last step for the usual boot sequence.
5. If the executable file /linuxrc is present in the initial root
file-system, /linuxrc is
executed with UID 0. (The file /linuxrc must have executable
permission. The file /linuxrc can be any valid executable,
including a shell script.)
6. If /linuxrc is not
executed or when /linuxrc
terminates, the normal root file system is mounted. (If
/linuxrc exits with any
file-systems mounted on the initial root file-system,
then the behavior of the kernel is UNSPECIFIED. See the NOTES section for the current kernel
behavior.)
7. If the normal root file has directory /initrd, device /dev/ram0 is moved from / to /initrd. Otherwise if directory
/initrd does not exist
device /dev/ram0 is
unmounted. (When moved from / to /initrd, /dev/ram0 is not unmounted and
therefore processes can remain running from /dev/ram0. If directory /initrd does not exist on the normal
root file-system and any processes remain running from
/dev/ram0 when /linuxrc exits, the behavior of the
kernel is UNSPECIFIED. See
the NOTES section for the
current kernel behavior.)
8. The usual boot sequence (e.g. invocation of
/sbin/init) is performed on
the normal root file system.
OPTIONS
The following boot loader options when used with
initrd, affect the
kernel's boot-up operation:
initrd=filename-
Specifies the file to load as the contents of
/dev/initrd. For
LOADLIN this is a command
line option. For LILO you
have to use this command in the LILO configuration file /etc/lilo.config. The filename
specified with this option will typically be a gzipped
file-system image.
noinitrd-
This boot time option disables the two phase boot-up
operation. The kernel performs the usual boot sequence
as if /dev/initrd was not
initialized. With this option, any contents of
/dev/initrd loaded into
memory by the boot loader contents are preserved. This
option permits the contents of /dev/initrd to be any data and need
not be limited to a file system image. However, device
/dev/initrd is read-only
and can be read only one time after system startup.
root=device-name-
Specifies the device to be used as the normal root
file system. For LOADLIN
this is a command line option. For LILO this is a boot time option or
can be used as an option line in the LILO configuration file /etc/lilo.config. The device
specified by the this option must be a mountable device
having a suitable root file-system.
CHANGING THE NORMAL ROOT FILE SYSTEM
By default, the kernel's settings (e.g. set in the kernel
file with rdev(8) 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 nfs_root_name and nfs_root_addrs boot options
to give the NFS settings. For more information on NFS-mounted
root see the kernel documentation file nfsroot.txt. For more
information on setting the root file system also see the
LILO and LOADLIN documentation.
It is also possible for the /linuxrc executable to change the normal
root device. For /linuxrc to
change the normal root device, /proc must be mounted. After mounting
/proc, /linuxrc changes the normal root device by
writing into the proc files /proc/sys/kernel/real-root-dev,
/proc/sys/kernel/nfs-root-name,
and /proc/sys/kernel/nfs-root-addrs. For a
physical root device, the root device is changed by having
/linuxrc write the new root
file system device number into /proc/sys/kernel/real-root-dev. For a NFS
root file system, the root device is changed by having
/linuxrc write the NFS setting
into files /proc/sys/kernel/nfs-root-name and
/proc/sys/kernel/nfs-root-addrs
and then writing 0xff (e.g. the pseudo-NFS-device number)
into file /proc/sys/kernel/real-root-dev. For
example, the following shell command line would change the
normal root device to /dev/hdb1:
For a NFS example, the following shell command lines would
change the normal root device to the NFS directory
/var/nfsroot on a local
networked NFS server with IP number 193.8.232.7 for a system
with IP number 193.8.232.7 and named 'idefix':
Note: The use of
/proc/sys/kernel/real-root-dev
to change the root file system is obsolete. See the kernel
source file Documentation/initrd.txt as
well as pivot_root(2) and pivot_root(8) for
information on the modern method of changing the root file
system.
USAGE
The main motivation for implementing initrd was to allow for
modular kernel configuration at system installation.
A possible system installation scenario is as follows:
1. The loader program boots from floppy or other media
with a minimal kernel (e.g. support for /dev/ram, /dev/initrd, and the ext2 file-system)
and loads /dev/initrd with
a gzipped version of the initial file-system.
2. The executable /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.
3. The executable /linuxrc loads the necessary modules
from the initial root file-system.
4. The executable /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.)
5. The executable /linuxrc sets /proc/sys/kernel/real-root-dev, unmount
/proc, the normal root file
system and any other file systems it has mounted, and
then terminates.
6. The kernel then mounts the normal root file
system.
7. Now that the file system is accessible and intact,
the boot loader can be installed.
8. The boot loader is configured to load into
/dev/initrd a file system
with the set of modules that was used to bring up the
system. (e.g. Device /dev/ram0 can be modified, then
unmounted, and finally, the image is written from
/dev/ram0 to a file.)
9. The system is now bootable and additional
installation tasks can be performed.
The key role of /dev/initrd
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.
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 /linuxrc file or a file executed by
/linuxrc would be
different.
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
/dev/initrd can use a dialog
and/or auto-detection followed by a possible sanity
check.
Last but not least, Linux distributions on CD-ROM may use
initrd for easy
installation from the CD-ROM. The distribution can use
LOADLIN to directly load
/dev/initrd from CD-ROM without
the need of any floppies. The distribution could also use a
LILO boot floppy and then
bootstrap a bigger ram disk via /dev/initrd from the CD-ROM.
CONFIGURATION
The /dev/initrd is a
read-only block device assigned major number 1 and minor
number 250. Typically /dev/initrd is owned by root.disk with mode 0400
(read access by root only). If the Linux system does not have
/dev/initrd already created, it
can be created with the following commands:
Also, support for both "RAM disk" and "Initial RAM disk"
(e.g. CONFIG_BLK_DEV_RAM=y and
CONFIG_BLK_DEV_INITRD=y )
support must be compiled directly into the Linux kernel to
use /dev/initrd. When using
/dev/initrd, the RAM disk
driver cannot be loaded as a module.
NOTES
1. With the current kernel, any file systems that remain
mounted when /dev/ram0 is moved
from / to /initrd continue to be accessible. However,
the /proc/mounts entries are
not updated.
2. With the current kernel, if directory /initrd does not exist, then /dev/ram0 will NOT be fully unmounted if
/dev/ram0 is used by any
process or has any file-system mounted on it. If /dev/ram0 is NOT fully unmounted, then
/dev/ram0 will remain in
memory.
3. Users of /dev/initrd
should not depend on the behavior give in the above notes.
The behavior may change in future versions of the Linux
kernel.
AUTHOR
The kernel code for device initrd was written by Werner
Almesberger <almesber@lrc.epfl.ch> and Hans Lermen
<lermen@elserv.ffm.fgan.de>. The code for initrd was added to the
baseline Linux kernel in development version 1.3.73.
SEE ALSO
chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)
The documentation file initrd.txt in the kernel
source package, the LILO documentation, the LOADLIN
documentation, the SYSLINUX documentation.
This man-page is Copyright (C) 1997 John S. Kallal
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date. The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein. The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.
Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and author(s) of this work.
If the you wish to distribute versions of this work under other
conditions than the above, please contact the author(s) at the following
for permission:
John S. Kallal -
email: <kallal@voicenet.com>
mail: 518 Kerfoot Farm RD, Wilmington, DE 19803-2444, USA
phone: (302)654-5478
$Id: initrd.4,v 0.9 1997/11/07 05:05:32 kallal Exp kallal $
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