Using the initial RAM disk (initrd)¶
initrd provides the capability to load a RAM disk by the boot loader. This RAM disk can then be mounted as the root file system and programs can be run from it. Afterwards, a new root file system can be mounted from a different device. The previous root (from initrd) is then moved to a directory and can be subsequently unmounted.
initrd is mainly designed to allow system startup to occur in two phases, where the kernel comes up with a minimum set of compiled-in drivers, and where additional modules are loaded from initrd.
This document gives a brief overview of the use of initrd. A more detailed discussion of the boot process can be found in 1.
When using initrd, the system typically boots as follows:
the boot loader loads the kernel and the initial RAM disk
the kernel converts initrd into a "normal" RAM disk and frees the memory used by initrd
if the root device is not
/dev/ram0, the old (deprecated) change_root procedure is followed. see the "Obsolete root change mechanism" section below.
root device is mounted. if it is
/dev/ram0, the initrd image is then mounted as root
/sbin/init is executed (this can be any valid executable, including shell scripts; it is run with uid 0 and can do basically everything init can do).
init mounts the "real" root file system
init places the root file system at the root directory using the pivot_root system call
init execs the
/sbin/initon the new root filesystem, performing the usual boot sequence
the initrd file system is removed
Note that changing the root directory does not involve unmounting it. It is therefore possible to leave processes running on initrd during that procedure. Also note that file systems mounted under initrd continue to be accessible.
Boot command-line options¶
initrd adds the following new options:
initrd=<path> (e.g. LOADLIN) Loads the specified file as the initial RAM disk. When using LILO, you have to specify the RAM disk image file in /etc/lilo.conf, using the INITRD configuration variable. noinitrd initrd data is preserved but it is not converted to a RAM disk and the "normal" root file system is mounted. initrd data can be read from /dev/initrd. Note that the data in initrd can have any structure in this case and doesn't necessarily have to be a file system image. This option is used mainly for debugging. Note: /dev/initrd is read-only and it can only be used once. As soon as the last process has closed it, all data is freed and /dev/initrd can't be opened anymore. root=/dev/ram0 initrd is mounted as root, and the normal boot procedure is followed, with the RAM disk mounted as root.
Compressed cpio images¶
Recent kernels have support for populating a ramdisk from a compressed cpio archive. On such systems, the creation of a ramdisk image doesn't need to involve special block devices or loopbacks; you merely create a directory on disk with the desired initrd content, cd to that directory, and run (as an example):
find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
Examining the contents of an existing image file is just as simple:
mkdir /tmp/imagefile cd /tmp/imagefile gzip -cd /boot/imagefile.img | cpio -imd --quiet
First, a directory for the initrd file system has to be created on the "normal" root file system, e.g.:
# mkdir /initrd
The name is not relevant. More details can be found on the pivot_root(2) man page.
If the root file system is created during the boot procedure (i.e. if
you're building an install floppy), the root file system creation
procedure should create the
If initrd will not be mounted in some cases, its content is still accessible if the following device has been created:
# mknod /dev/initrd b 1 250 # chmod 400 /dev/initrd
Second, the kernel has to be compiled with RAM disk support and with support for the initial RAM disk enabled. Also, at least all components needed to execute programs from initrd (e.g. executable format and file system) must be compiled into the kernel.
Third, you have to create the RAM disk image. This is done by creating a file system on a block device, copying files to it as needed, and then copying the content of the block device to the initrd file. With recent kernels, at least three types of devices are suitable for that:
a floppy disk (works everywhere but it's painfully slow)
a RAM disk (fast, but allocates physical memory)
a loopback device (the most elegant solution)
We'll describe the loopback device method:
make sure loopback block devices are configured into the kernel
create an empty file system of the appropriate size, e.g.:# dd if=/dev/zero of=initrd bs=300k count=1 # mke2fs -F -m0 initrd
(if space is critical, you may want to use the Minix FS instead of Ext2)
mount the file system, e.g.:# mount -t ext2 -o loop initrd /mnt
create the console device:# mkdir /mnt/dev # mknod /mnt/dev/console c 5 1
copy all the files that are needed to properly use the initrd environment. Don't forget the most important file,
/sbin/initpermissions must include "x" (execute).
correct operation the initrd environment can frequently be tested even without rebooting with the command:# chroot /mnt /sbin/init
This is of course limited to initrds that do not interfere with the general system state (e.g. by reconfiguring network interfaces, overwriting mounted devices, trying to start already running demons, etc. Note however that it is usually possible to use pivot_root in such a chroot'ed initrd environment.)
unmount the file system:# umount /mnt
the initrd is now in the file "initrd". Optionally, it can now be compressed:# gzip -9 initrd
For experimenting with initrd, you may want to take a rescue floppy and
only add a symbolic link from
/bin/sh. Alternatively, you
can try the experimental newlib environment 2 to create a small
Finally, you have to boot the kernel and load initrd. Almost all Linux boot loaders support initrd. Since the boot process is still compatible with an older mechanism, the following boot command line parameters have to be given:
(rw is only necessary if writing to the initrd file system.)
With LOADLIN, you simply execute:
LOADLIN <kernel> initrd=<disk_image>
LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw
With LILO, you add the option
INITRD=<path> to either the global section
or to the section of the respective kernel in
/etc/lilo.conf, and pass
the options using APPEND, e.g.:
image = /bzImage initrd = /boot/initrd.gz append = "root=/dev/ram0 rw"
For other boot loaders, please refer to the respective documentation.
Now you can boot and enjoy using initrd.
Changing the root device¶
When finished with its duties, init typically changes the root device and proceeds with starting the Linux system on the "real" root device.
- The procedure involves the following steps:
mounting the new root file system
turning it into the root file system
removing all accesses to the old (initrd) root file system
unmounting the initrd file system and de-allocating the RAM disk
Mounting the new root file system is easy: it just needs to be mounted on a directory under the current root. Example:
# mkdir /new-root # mount -o ro /dev/hda1 /new-root
The root change is accomplished with the pivot_root system call, which
is also available via the
pivot_root utility (see pivot_root(8)
pivot_root is distributed with util-linux version 2.10h or higher
pivot_root moves the current root to a directory under the new
root, and puts the new root at its place. The directory for the old root
must exist before calling
# cd /new-root # mkdir initrd # pivot_root . initrd
Now, the init process may still access the old root via its executable, shared libraries, standard input/output/error, and its current root directory. All these references are dropped by the following command:
# exec chroot . what-follows <dev/console >dev/console 2>&1
Where what-follows is a program under the new root, e.g.
If the new root file system will be used with udev and has no valid
/dev directory, udev must be initialized before invoking chroot in order
Note: implementation details of pivot_root may change with time. In order to ensure compatibility, the following points should be observed:
before calling pivot_root, the current directory of the invoking process should point to the new root directory
use . as the first argument, and the _relative_ path of the directory for the old root as the second argument
a chroot program must be available under the old and the new root
chroot to the new root afterwards
use relative paths for dev/console in the exec command
Now, the initrd can be unmounted and the memory allocated by the RAM disk can be freed:
# umount /initrd # blockdev --flushbufs /dev/ram0
It is also possible to use initrd with an NFS-mounted root, see the pivot_root(8) man page for details.
The main motivation for implementing initrd was to allow for modular kernel configuration at system installation. The procedure would work as follows:
system boots from floppy or other media with a minimal kernel (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and loads initrd
/sbin/initdetermines what is needed to (1) mount the "real" root FS (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.
/sbin/initloads the necessary kernel modules
/sbin/initcreates and populates the root file system (this doesn't have to be a very usable system yet)
pivot_rootto change the root file system and execs - via chroot - a program that continues the installation
the boot loader is installed
the boot loader is configured to load an initrd with the set of modules that was used to bring up the system (e.g.
/initrdcan be modified, then unmounted, and finally, the image is written from
/dev/rd/0to a file)
now the system is bootable and additional installation tasks can be performed
The key role of initrd here is to re-use the configuration data during normal system operation without requiring the use of a bloated "generic" kernel or re-compiling or re-linking the kernel.
A second scenario is for installations where Linux runs on systems with
different hardware configurations in a single administrative domain. In
such cases, it is desirable to generate 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, a common initrd could be
generated with all the necessary modules. Then, only
/sbin/init or a file
read by it would have to be different.
A third scenario is more convenient recovery disks, because information like the location of the root FS partition doesn't have to be provided at boot time, but the system loaded from initrd can invoke a user-friendly dialog and it can also perform some sanity checks (or even some form of auto-detection).
Last not least, CD-ROM distributors may use it for better installation
from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
via initrd from CD; or by booting via a loader like
LOADLIN or directly
from the CD-ROM, and loading the RAM disk from CD without need of
Obsolete root change mechanism¶
The following mechanism was used before the introduction of pivot_root. Current kernels still support it, but you should _not_ rely on its continued availability.
It works by mounting the "real" root device (i.e. the one set with rdev
in the kernel image or with root=... at the boot command line) as the
root file system when linuxrc exits. The initrd file system is then
unmounted, or, if it is still busy, moved to a directory
such a directory exists on the new root file system.
In order to use this mechanism, you do not have to specify the boot command options root, init, or rw. (If specified, they will affect the real root file system, not the initrd environment.)
If /proc is mounted, the "real" root device can be changed from within linuxrc by writing the number of the new root FS device to the special file /proc/sys/kernel/real-root-dev, e.g.:
# echo 0x301 >/proc/sys/kernel/real-root-dev
Note that the mechanism is incompatible with NFS and similar file systems.
This old, deprecated mechanism is commonly called
the new, supported mechanism is called
Mixed change_root and pivot_root mechanism¶
In case you did not want to use
root=/dev/ram0 to trigger the pivot_root
mechanism, you may create both
/sbin/init in your initrd
/linuxrc would contain only the following:
#! /bin/sh mount -n -t proc proc /proc echo 0x0100 >/proc/sys/kernel/real-root-dev umount -n /proc
Once linuxrc exited, the kernel would mount again your initrd as root,
this time executing
/sbin/init. Again, it would be the duty of this init
to build the right environment (maybe using the
root= device passed on
the cmdline) before the final execution of the real
Almesberger, Werner; "Booting Linux: The History and the Future" https://www.almesberger.net/cv/papers/ols2k-9.ps.gz
newlib package (experimental), with initrd example https://www.sourceware.org/newlib/
util-linux: Miscellaneous utilities for Linux https://www.kernel.org/pub/linux/utils/util-linux/