NAME¶
initrd - boot loader initialized RAM disk
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:
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'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.
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 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.
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 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 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.
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 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.
FILES¶
/dev/initrd
/dev/ram0
/linuxrc
/initrd
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.
SEE ALSO¶
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
COLOPHON¶
This page is part of release 3.44 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/.