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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+Using the initial RAM disk (initrd)
+===================================
+
+Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
+Hans 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 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 [#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 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
+
+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. if
+you're building an install floppy), the root file system creation
+procedure should create the ``/initrd`` directory.
+
+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:
+
+ 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 and
+only add a symbolic link from ``/sbin/init`` to ``/bin/sh``. Alternatively, you
+can try the experimental newlib environment [#f2]_ to create a small
+initrd.
+
+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::
+
+  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 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"
+
+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 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 :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 new
+root, and puts the new root at its place. The directory for the old root
+must 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 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. ``/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 order
+to provide ``/dev/console``.
+
+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
+:manpage:`pivot_root(8)` man page for details.
+
+
+Usage scenarios
+---------------
+
+The main motivation for implementing initrd was to allow for modular
+kernel configuration at system installation. The procedure would work
+as 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 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
+floppies.
+
+
+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 ``/initrd``, if
+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 ``change_root``, while
+the 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_root
+mechanism, you may create both ``/linuxrc`` and ``/sbin/init`` in your initrd
+image.
+
+``/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 ``/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/