NAME
initrd - boot loader initialized RAM disk
DESCRIPTION
The special file /dev/initrd is a read-only block device.
Device /dev/initrd is a RAM disk that is initialized (e.g.
loaded) by the boot loader before the kernel is started.
The kernel then can use the the block device /dev/initrd's
contents for a two phased 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 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 ini-
tial 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 ter-
minates, the normal root file system is mounted. (If
/linuxrc exits with any file-systems mounted on the ini-
tial 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 has directory /initrd, device
/dev/ram0 is moved from / to /initrd. Otherwise if direc-
tory /initrd does not exist 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 time 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 or compiled into the kernel file), or the
boot loader option setting is used for the normal root file
systems. For a 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 NSF-mounted
root see the kernel documentation file nfsroot.txt. For
more information on setting the root file system also see
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 a
NSF root file system, the root device is changed by having
/linuxrc write the NSF 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 exam-
ple, the following shell command line would change the nor-
mal root device to /dev/hdb1:
echo 0x365 >/proc/sys/kernel/real-root-dev
For a NSF example, the following shell command lines would
change the normal root device to the NSF directory
/var/nfsroot on a local networked NSF server with IP number
193.8.232.7 for a system with IP number 193.8.232.7 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
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 ter-
minates.
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 modi-
fied, 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 re-use 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 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 parti-
tion 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 distribu-
tion 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.
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 )
support 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.
FILES
/dev/initrd
/dev/ram0
/linuxrc
/initrd
SEE ALSO
chown(1), mknod(1), /dev/ram(4), freeramdisk(8), rdev(8),
The documentation file initrd.txt in the kernel source pack-
age, the LILO documentation, the LOADLIN documentation, the
SYSLINUX documentation.
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.
AUTHOR
The kernel code for device initrd was written by Werner
Almesberger <almesber@lrc.epfl.ch> and Hans Lermen
<lermen@elserv.ffm.fgan.de>. The code for initrd was added
to the baseline Linux kernel in development version 1.3.73.