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INITRD(4) Linux Programmer's Manual INITRD(4)
initrd - boot loader initialized RAM disk
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 fol-
lowing 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 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.
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 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 system 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 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.
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 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.
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 an 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
Documentation/filesystems/nfsroot.txt. For more information on
setting the root file system 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 file system device number into
/proc/sys/kernel/real-root-dev. For an 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:
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
file system is obsolete. See the Linux 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.
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 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 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.
/dev/initrd
/dev/ram0
/linuxrc
/initrd
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.
chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)
Documentation/initrd.txt in the Linux kernel source tree, the LILO
documentation, the LOADLIN documentation, the SYSLINUX documentation
This page is part of release 3.51 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2010-09-04 INITRD(4)
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