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NAME | CONFIGURATION | DESCRIPTION | FILES | NOTES | SEE ALSO | COLOPHON |
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initrd(4) Kernel Interfaces Manual initrd(4)
initrd - boot loader initialized RAM disk
/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.
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'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'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:
(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 the device /dev/ram0 as the
initial root filesystem.
(4) 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.
(5) 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.)
(6) 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 is UNSPECIFIED. See the NOTES
section for the current kernel behavior.)
(7) 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 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 filesystem.
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 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.
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 this option must
be a mountable device having a suitable root filesystem.
Changing the normal root filesystem
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 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
Note: The use of /proc/sys/kernel/real-root-dev to change the root
filesystem is obsolete. See the Linux kernel source file
Documentation/admin-guide/initrd.rst (or Documentation/initrd.txt
before Linux 4.10) as well as pivot_root(2) and pivot_root(8) for
information on the modern method of changing the root filesystem.
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 filesystem) and loads /dev/initrd with a gzipped
version of the initial filesystem.
(2) 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.
(3) The executable /linuxrc loads the necessary modules from the
initial root filesystem.
(4) 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.)
(5) The executable /linuxrc sets /proc/sys/kernel/real-root-dev,
unmounts /proc, the normal root filesystem and any other
filesystems it has mounted, and then terminates.
(6) The kernel then mounts the normal root filesystem.
(7) Now that the filesystem is accessible and intact, the boot
loader can be installed.
(8) 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.)
(9) 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.
/dev/initrd
/dev/ram0
/linuxrc
/initrd
• 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 given in
the above notes. The behavior may change in future versions of
the Linux kernel.
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
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-02-02. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.10 2024-06-15 initrd(4)
Pages that refer to this page: pivot_root(2)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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