initrd
INITRD(4) Linux Programmer’s Manual INITRD(4)
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 ini-
tialized 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 con-
tents into memory.
2. On kernel startup, the kernel uncompresses and copies the con-
tents 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 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 nor-
mal 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 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 run-
ning 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 UNSPEC-
IFIED. See the NOTES section for the current kernel behavior.)
8. The usual boot sequence (e.g. invocation of /sbin/init) is per-
formed 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.con-
fig. 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 set-
ting is used for the normal root file systems. For a NFS-mounted nor-
mal root file system, one has to use the nfs_root_name and
nfs_root_addrs boot options to give the NFS settings. For more infor-
mation 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 hav-
ing /linuxrc write the new root file system 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 com-
mand 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 net-
worked NFS server with IP number 193.8.232.7 for a system with IP num-
ber 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 mini-
mal 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 ini-
tial root file-system.
4. The executable /linuxrc creates and populates the root file sys-
tem. (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 sys-
tems 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 re-use 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 net-
work. 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 informa-
tion 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 flop-
pies. 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 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 ) support must be com-
piled 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), ram(4), freeramdisk(8), rdev(8), The documentation
file initrd.txt in the kernel source package, 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. How-
ever, 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 pro-
cess 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 develop-
ment version 1.3.73.
Linux 2.0 1997-11-06 INITRD(4)
