Using the initial RAM disk (initrd)=================================== Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> andHans Lermen <lermen@fgan.de>  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 programscan be run from it. Afterwards, a new root file system can be mountedfrom a different device. The previous root (from initrd) is then movedto 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, andwhere additional modules are loaded from initrd. This document gives a brief overview of the use of initrd. A more detaileddiscussion of the boot process can be found in [#f1]_.  Operation--------- When using initrd, the system typically boots as follows:   1) the boot loader loads the kernel and the initial RAM disk  2) the kernel converts initrd into a "normal" RAM disk and     frees the memory used by initrd  3) if the root device is not ``/dev/ram0``, the old (deprecated)     change_root procedure is followed. see the "Obsolete root change     mechanism" section below.  4) root device is mounted. if it is ``/dev/ram0``, the initrd image is     then mounted as root  5) /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).  6) init mounts the "real" root file system  7) init places the root file system at the root directory using the     pivot_root system call  8) init execs the ``/sbin/init`` on the new root filesystem, performing     the usual boot sequence  9) 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 thatprocedure. Also note that file systems mounted under initrd continue tobe 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 cpioarchive. On such systems, the creation of a ramdisk image doesn't need toinvolve special block devices or loopbacks; you merely create a directory ondisk with the desired initrd content, cd to that directory, and run (as anexample):: 	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 Installation------------ 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:manpage:`pivot_root(2)` man page. If the root file system is created during the boot procedure (i.e. ifyou're building an install floppy), the root file system creationprocedure should create the ``/initrd`` directory. If initrd will not be mounted in some cases, its content is stillaccessible 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 withsupport for the initial RAM disk enabled. Also, at least all componentsneeded to execute programs from initrd (e.g. executable format and filesystem) must be compiled into the kernel. Third, you have to create the RAM disk image. This is done by creating afile system on a block device, copying files to it as needed, and thencopying the content of the block device to the initrd file. With recentkernels, 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:  1) make sure loopback block devices are configured into the kernel 2) 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) 3) mount the file system, e.g.:: 	# mount -t ext2 -o loop initrd /mnt  4) create the console device::     # mkdir /mnt/dev    # mknod /mnt/dev/console c 5 1  5) copy all the files that are needed to properly use the initrd    environment. Don't forget the most important file, ``/sbin/init``     .. note:: ``/sbin/init`` permissions must include "x" (execute).  6) 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.) 7) unmount the file system:: 	# umount /mnt  8) 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 andonly add a symbolic link from ``/sbin/init`` to ``/bin/sh``. Alternatively, youcan try the experimental newlib environment [#f2]_ to create a smallinitrd. Finally, you have to boot the kernel and load initrd. Almost all Linuxboot loaders support initrd. Since the boot process is still compatiblewith an older mechanism, the following boot command line parametershave to be given::   root=/dev/ram0 rw (rw is only necessary if writing to the initrd file system.) With LOADLIN, you simply execute::      LOADLIN <kernel> initrd=<disk_image> e.g.:: 	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 sectionor to the section of the respective kernel in ``/etc/lilo.conf``, and passthe options using APPEND, e.g.::   image = /bzImage    initrd = /boot/initrd.gz    append = "root=/dev/ram0 rw" and run ``/sbin/lilo`` 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 deviceand 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 ona 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, whichis also available via the ``pivot_root`` utility (see :manpage:`pivot_root(8)`man page; ``pivot_root`` is distributed with util-linux version 2.10h or higher[#f3]_). ``pivot_root`` moves the current root to a directory under the newroot, and puts the new root at its place. The directory for the old rootmust exist before calling ``pivot_root``. Example:: 	# cd /new-root	# mkdir initrd	# pivot_root . initrd Now, the init process may still access the old root via itsexecutable, shared libraries, standard input/output/error, and itscurrent root directory. All these references are dropped by thefollowing command:: 	# exec chroot . what-follows <dev/console >dev/console 2>&1 Where what-follows is a program under the new root, e.g. ``/sbin/init``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 orderto provide ``/dev/console``. Note: implementation details of pivot_root may change with time. In orderto 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 RAMdisk can be freed:: 	# umount /initrd	# blockdev --flushbufs /dev/ram0 It is also possible to use initrd with an NFS-mounted root, see the:manpage:`pivot_root(8)` man page for details.  Usage scenarios--------------- The main motivation for implementing initrd was to allow for modularkernel configuration at system installation. The procedure would workas follows:   1) 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  2) ``/sbin/init`` determines 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.  3) ``/sbin/init`` loads the necessary kernel modules  4) ``/sbin/init`` creates and populates the root file system (this doesn't     have to be a very usable system yet)  5) ``/sbin/init`` invokes ``pivot_root`` to change the root file system and     execs - via chroot - a program that continues the installation  6) the boot loader is installed  7) the boot loader is configured to load an initrd with the set of     modules that was used to bring up the system (e.g. ``/initrd`` can be     modified, then unmounted, and finally, the image is written from     ``/dev/ram0`` or ``/dev/rd/0`` to a file)  8) now the system is bootable and additional installation tasks can be     performed The key role of initrd here is to reuse the configuration data duringnormal 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 withdifferent hardware configurations in a single administrative domain. Insuch cases, it is desirable to generate only a small set of kernels(ideally only one) and to keep the system-specific part of configurationinformation as small as possible. In this case, a common initrd could begenerated with all the necessary modules. Then, only ``/sbin/init`` or a fileread by it would have to be different. A third scenario is more convenient recovery disks, because informationlike the location of the root FS partition doesn't have to be provided atboot time, but the system loaded from initrd can invoke a user-friendlydialog and it can also perform some sanity checks (or even some form ofauto-detection). Last not least, CD-ROM distributors may use it for better installationfrom CD, e.g. by using a boot floppy and bootstrapping a bigger RAM diskvia initrd from CD; or by booting via a loader like ``LOADLIN`` or directlyfrom the CD-ROM, and loading the RAM disk from CD without need offloppies.  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 itscontinued availability. It works by mounting the "real" root device (i.e. the one set with rdevin the kernel image or with root=... at the boot command line) as theroot file system when linuxrc exits. The initrd file system is thenunmounted, or, if it is still busy, moved to a directory ``/initrd``, ifsuch a directory exists on the new root file system. In order to use this mechanism, you do not have to specify the bootcommand options root, init, or rw. (If specified, they will affectthe real root file system, not the initrd environment.) If /proc is mounted, the "real" root device can be changed from withinlinuxrc by writing the number of the new root FS device to the specialfile /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 filesystems. This old, deprecated mechanism is commonly called ``change_root``, whilethe new, supported mechanism is called ``pivot_root``.  Mixed change_root and pivot_root mechanism------------------------------------------ In case you did not want to use ``root=/dev/ram0`` to trigger the pivot_rootmechanism, you may create both ``/linuxrc`` and ``/sbin/init`` in your initrdimage. ``/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 initto build the right environment (maybe using the ``root= device`` passed onthe cmdline) before the final execution of the real ``/sbin/init``.  Resources--------- .. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future"    https://www.almesberger.net/cv/papers/ols2k-9.ps.gz.. [#f2] newlib package (experimental), with initrd example    https://www.sourceware.org/newlib/.. [#f3] util-linux: Miscellaneous utilities for Linux    https://www.kernel.org/pub/linux/utils/util-linux/