Section (4) initrd
Name
initrd — boot loader initialized RAM disk
CONFIGURATION
/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:
mknod −m 400 /dev/initrd b 1 250 chown root:disk /dev/initrd
Also, support for both RAM disk and Initial RAM disk
(e.g., CONFIG_BLK_DEV_RAM=y
and
CONFIG_BLK_DEV_INITRD=y
) 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.
DESCRIPTION
The special file /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 /dev/initrd
_zsingle_quotesz_s contents for a two-phase
system boot-up.
In the first boot-up phase, the kernel starts up and
mounts an initial root filesystem 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_zsingle_quotesz_s contents. After loading the additional
modules, a new root filesystem (i.e., the normal root
filesystem) is mounted from a different device.
Boot-up operation
When booting up with initrd
, the system boots as follows:
-
The boot loader loads the kernel program and
/dev/initrd
_zsingle_quotesz_s contents into memory. -
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
. -
The kernel then read-write mounts the device
/dev/ram0
as the initial root filesystem. -
If the indicated normal root filesystem is also the initial root filesystem (e.g.,
/dev/ram0
) then the kernel skips to the last step for the usual boot sequence. -
If the executable file
/linuxrc
is present in the initial root filesystem,/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.) -
If
/linuxrc
is not executed or when/linuxrc
terminates, the normal root filesystem is mounted. (If/linuxrc
exits with any filesystems mounted on the initial root filesystem, then the behavior of the kernel isUNSPECIFIED
. See the NOTES section for the current kernel behavior.) -
If the normal root filesystem has a directory
/initrd
, the device/dev/ram0
is moved from/
to/initrd
. Otherwise, if the directory/initrd
does not exist, the 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 filesystem and any processes remain running from/dev/ram0
when/linuxrc
exits, the behavior of the kernel isUNSPECIFIED
. See the NOTES section for the current kernel behavior.) -
The usual boot sequence (e.g., invocation of
/sbin/init
) is performed on the normal root filesystem.
Options
The following boot loader options, when used with
initrd
, affect the kernel_zsingle_quotesz_s
boot-up operation:
initrd=
filename
-
Specifies the file to load as the contents of
/dev/initrd
. ForLOADLIN
this is a command-line option. ForLILO
you have to use this command in theLILO
configuration file/etc/lilo.config
. The filename specified with this option will typically be a gzipped filesystem image. noinitrd
-
This boot 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 filesystem 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 filesystem. For
LOADLIN
this is a command-line option. ForLILO
this is a boot time option or can be used as an option line in theLILO
configuration file/etc/lilo.config
. The device specified by the this option must be a mountable device having a suitable root filesystem.
Changing the normal root filesystem
By default, the kernel_zsingle_quotesz_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 filesystems. For an NFS-mounted normal root
filesystem, 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 Documentation/filesystems/nfs/nfsroot.txt
(or Documentation/filesystems/nfsroot.txt
before Linux 2.6.33). For more information on setting the
root filesystem see also 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
filesystem device number into /proc/sys/kernel/real-root-dev
. For an
NFS root filesystem, 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
:
echo 0x365 >/proc/sys/kernel/real-root-dev
For an 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 193.8.232.2 and named idefix:
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
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Note |
---|---|
The use of |
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:
-
The loader program boots from floppy or other media with a minimal kernel (e.g., support for
/dev/ram
,/dev/initrd
, and the ext2 filesystem) and loads/dev/initrd
with a gzipped version of the initial filesystem. -
The executable
/linuxrc
determines what is needed to (1) mount the normal root filesystem (i.e., device type, device drivers, filesystem) 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. -
The executable
/linuxrc
loads the necessary modules from the initial root filesystem. -
The executable
/linuxrc
creates and populates the root filesystem. (At this stage the normal root filesystem does not have to be a completed system yet.) -
The executable
/linuxrc
sets/proc/sys/kernel/real-root-dev
, unmount/proc
, the normal root filesystem and any other filesystems it has mounted, and then terminates. -
The kernel then mounts the normal root filesystem.
-
Now that the filesystem is accessible and intact, the boot loader can be installed.
-
The boot loader is configured to load into
/dev/initrd
a filesystem 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.) -
The system is now bootable and additional installation tasks can be performed.
The key role of /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.
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
filesystem 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.
NOTES
-
With the current kernel, any filesystems that remain mounted when
/dev/ram0
is moved from/
to/initrd
continue to be accessible. However, the/proc/mounts
entries are not updated. -
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 filesystem mounted on it. If/dev/ram0
is not fully unmounted, then/dev/ram0
will remain in memory. -
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.
SEE ALSO
chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)
Documentation/admin−guide/initrd.rst
(or Documentation/initrd.txt
before Linux 4.10) in the Linux kernel source tree, the LILO
documentation, the LOADLIN documentation, the SYSLINUX
documentation
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
This man-page is Copyright (C) 1997 John S. Kallal %%%LICENSE_START(VERBATIM) 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. %%%LICENSE_END 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: <kallalvoicenet.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 $ |