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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 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 mem-
ory.
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 initial root file-system,
/linuxrc is executed with uid 0. (The file /linuxrc must have executable permis-
sion. 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 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 there-
fore 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 com-
piled into the kernel file), or the boot loader option setting is used for the nor-
mal 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 NFS-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 mount-
ing /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/ker-
nel/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/ker-
nel/real-root-dev. For a NFS root file system, the root device is changed by hav-
ing /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
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 config-
uration 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 distribu-
tion 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 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 partition is not needed at boot time, the system
loaded from /dev/initrd can use a dialog and/or auto-detection followed by a possi-
ble sanity check.
Last but not least, Linux distributions on CD-ROM may use initrd for easy installa-
tion from the CD-ROM. The distribution can use LOADLIN to directly load /dev/ini-
trd 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 num-
ber 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. CON-
FIG_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), ram(4), freeramdisk(8), rdev(8), The documentation file ini-
trd.txt in the kernel source package, the LILO documentation, the LOADLIN documen-
tation, 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 <almes-
ber AT lrc.ch> and Hans Lermen <lermen AT elserv.de>. The code for initrd
was added to the baseline Linux kernel in development version 1.3.73.
Linux 2.0 1997-11-06 INITRD(4)
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