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NAME | CONFIGURATION | DESCRIPTION | FILES | NOTES | SEE ALSO | COLOPHON |
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INITRD(4) Linux Programmer's 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., CON‐
FIG_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
the 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 Documenta‐
tion/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 ini‐
tial 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,
unmount /proc, the normal root filesystem and any other filesys‐
tems 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 filesys‐
tem 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 configura‐
tion 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 informa‐
tion 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
1. 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.
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 filesystem 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.
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 release 4.15 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/.
Linux 2017-09-15 INITRD(4)
Pages that refer to this page: pivot_root(2)
Copyright and license for this manual page
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