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relacelcnal NAME

initrd - boot loader initialized RAM disk


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 con-
tents. 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 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
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 termi-
nates, 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
directory /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.


The following boot loader options when used with initrd,
affect the kernel's boot-up operation 
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.
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/ini-
trd is read-only and can be read only one time
after system startup.
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 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 nfsrootname and nfsrootaddrs boot
options to give the NFS settings. For more information on
NFS-mounted root see the kernel documentation file nfs-
root.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 sys-
tem device number into /proc/sys/kernel/real-root-dev.
For a 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 a 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 num-
ber for a system with IP number
and named 'idefix' 
echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
echo \
echo 255 >/proc/sys/kernel/real-root-dev


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-
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 termi-
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/ini-
trd 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 installa-
tion 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 desir-
able 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 dif-
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 distri-
bution 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.


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 com-
mknod -m 400 /dev/initrd b 1 250
chown root:disk /dev/initrd
Also, support for both "RAM disk" and "Initial RAM disk"
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.




chown(1), mknod(1), /dev/ram(4), freeramdisk(8), rdev(8),
The documentation file initrd.txt in the kernel source
package, the LILO documentation, the LOADLIN documenta-
tion, the SYSLINUX documentation.


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.


The kernel code for device initrd was written by Werner
Almesberger <> and Hans Lermen <ler->. The code for initrd was added to
the baseline Linux kernel in development version 1.3.73.