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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1	Using the initial RAM disk (initrd)
2	===================================
3	
4	Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
5	Hans Lermen <lermen@fgan.de>
6	
7	
8	initrd provides the capability to load a RAM disk by the boot loader.
9	This RAM disk can then be mounted as the root file system and programs
10	can be run from it. Afterwards, a new root file system can be mounted
11	from a different device. The previous root (from initrd) is then moved
12	to a directory and can be subsequently unmounted.
13	
14	initrd is mainly designed to allow system startup to occur in two phases,
15	where the kernel comes up with a minimum set of compiled-in drivers, and
16	where additional modules are loaded from initrd.
17	
18	This document gives a brief overview of the use of initrd. A more detailed
19	discussion of the boot process can be found in [#f1]_.
20	
21	
22	Operation
23	---------
24	
25	When using initrd, the system typically boots as follows:
26	
27	  1) the boot loader loads the kernel and the initial RAM disk
28	  2) the kernel converts initrd into a "normal" RAM disk and
29	     frees the memory used by initrd
30	  3) if the root device is not ``/dev/ram0``, the old (deprecated)
31	     change_root procedure is followed. see the "Obsolete root change
32	     mechanism" section below.
33	  4) root device is mounted. if it is ``/dev/ram0``, the initrd image is
34	     then mounted as root
35	  5) /sbin/init is executed (this can be any valid executable, including
36	     shell scripts; it is run with uid 0 and can do basically everything
37	     init can do).
38	  6) init mounts the "real" root file system
39	  7) init places the root file system at the root directory using the
40	     pivot_root system call
41	  8) init execs the ``/sbin/init`` on the new root filesystem, performing
42	     the usual boot sequence
43	  9) the initrd file system is removed
44	
45	Note that changing the root directory does not involve unmounting it.
46	It is therefore possible to leave processes running on initrd during that
47	procedure. Also note that file systems mounted under initrd continue to
48	be accessible.
49	
50	
51	Boot command-line options
52	-------------------------
53	
54	initrd adds the following new options::
55	
56	  initrd=<path>    (e.g. LOADLIN)
57	
58	    Loads the specified file as the initial RAM disk. When using LILO, you
59	    have to specify the RAM disk image file in /etc/lilo.conf, using the
60	    INITRD configuration variable.
61	
62	  noinitrd
63	
64	    initrd data is preserved but it is not converted to a RAM disk and
65	    the "normal" root file system is mounted. initrd data can be read
66	    from /dev/initrd. Note that the data in initrd can have any structure
67	    in this case and doesn't necessarily have to be a file system image.
68	    This option is used mainly for debugging.
69	
70	    Note: /dev/initrd is read-only and it can only be used once. As soon
71	    as the last process has closed it, all data is freed and /dev/initrd
72	    can't be opened anymore.
73	
74	  root=/dev/ram0
75	
76	    initrd is mounted as root, and the normal boot procedure is followed,
77	    with the RAM disk mounted as root.
78	
79	Compressed cpio images
80	----------------------
81	
82	Recent kernels have support for populating a ramdisk from a compressed cpio
83	archive. On such systems, the creation of a ramdisk image doesn't need to
84	involve special block devices or loopbacks; you merely create a directory on
85	disk with the desired initrd content, cd to that directory, and run (as an
86	example)::
87	
88		find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
89	
90	Examining the contents of an existing image file is just as simple::
91	
92		mkdir /tmp/imagefile
93		cd /tmp/imagefile
94		gzip -cd /boot/imagefile.img | cpio -imd --quiet
95	
96	Installation
97	------------
98	
99	First, a directory for the initrd file system has to be created on the
100	"normal" root file system, e.g.::
101	
102		# mkdir /initrd
103	
104	The name is not relevant. More details can be found on the
105	:manpage:`pivot_root(2)` man page.
106	
107	If the root file system is created during the boot procedure (i.e. if
108	you're building an install floppy), the root file system creation
109	procedure should create the ``/initrd`` directory.
110	
111	If initrd will not be mounted in some cases, its content is still
112	accessible if the following device has been created::
113	
114		# mknod /dev/initrd b 1 250
115		# chmod 400 /dev/initrd
116	
117	Second, the kernel has to be compiled with RAM disk support and with
118	support for the initial RAM disk enabled. Also, at least all components
119	needed to execute programs from initrd (e.g. executable format and file
120	system) must be compiled into the kernel.
121	
122	Third, you have to create the RAM disk image. This is done by creating a
123	file system on a block device, copying files to it as needed, and then
124	copying the content of the block device to the initrd file. With recent
125	kernels, at least three types of devices are suitable for that:
126	
127	 - a floppy disk (works everywhere but it's painfully slow)
128	 - a RAM disk (fast, but allocates physical memory)
129	 - a loopback device (the most elegant solution)
130	
131	We'll describe the loopback device method:
132	
133	 1) make sure loopback block devices are configured into the kernel
134	 2) create an empty file system of the appropriate size, e.g.::
135	
136		# dd if=/dev/zero of=initrd bs=300k count=1
137		# mke2fs -F -m0 initrd
138	
139	    (if space is critical, you may want to use the Minix FS instead of Ext2)
140	 3) mount the file system, e.g.::
141	
142		# mount -t ext2 -o loop initrd /mnt
143	
144	 4) create the console device::
145	
146	    # mkdir /mnt/dev
147	    # mknod /mnt/dev/console c 5 1
148	
149	 5) copy all the files that are needed to properly use the initrd
150	    environment. Don't forget the most important file, ``/sbin/init``
151	
152	    .. note:: ``/sbin/init`` permissions must include "x" (execute).
153	
154	 6) correct operation the initrd environment can frequently be tested
155	    even without rebooting with the command::
156	
157		# chroot /mnt /sbin/init
158	
159	    This is of course limited to initrds that do not interfere with the
160	    general system state (e.g. by reconfiguring network interfaces,
161	    overwriting mounted devices, trying to start already running demons,
162	    etc. Note however that it is usually possible to use pivot_root in
163	    such a chroot'ed initrd environment.)
164	 7) unmount the file system::
165	
166		# umount /mnt
167	
168	 8) the initrd is now in the file "initrd". Optionally, it can now be
169	    compressed::
170	
171		# gzip -9 initrd
172	
173	For experimenting with initrd, you may want to take a rescue floppy and
174	only add a symbolic link from ``/sbin/init`` to ``/bin/sh``. Alternatively, you
175	can try the experimental newlib environment [#f2]_ to create a small
176	initrd.
177	
178	Finally, you have to boot the kernel and load initrd. Almost all Linux
179	boot loaders support initrd. Since the boot process is still compatible
180	with an older mechanism, the following boot command line parameters
181	have to be given::
182	
183	  root=/dev/ram0 rw
184	
185	(rw is only necessary if writing to the initrd file system.)
186	
187	With LOADLIN, you simply execute::
188	
189	     LOADLIN <kernel> initrd=<disk_image>
190	
191	e.g.::
192	
193		LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw
194	
195	With LILO, you add the option ``INITRD=<path>`` to either the global section
196	or to the section of the respective kernel in ``/etc/lilo.conf``, and pass
197	the options using APPEND, e.g.::
198	
199	  image = /bzImage
200	    initrd = /boot/initrd.gz
201	    append = "root=/dev/ram0 rw"
202	
203	and run ``/sbin/lilo``
204	
205	For other boot loaders, please refer to the respective documentation.
206	
207	Now you can boot and enjoy using initrd.
208	
209	
210	Changing the root device
211	------------------------
212	
213	When finished with its duties, init typically changes the root device
214	and proceeds with starting the Linux system on the "real" root device.
215	
216	The procedure involves the following steps:
217	 - mounting the new root file system
218	 - turning it into the root file system
219	 - removing all accesses to the old (initrd) root file system
220	 - unmounting the initrd file system and de-allocating the RAM disk
221	
222	Mounting the new root file system is easy: it just needs to be mounted on
223	a directory under the current root. Example::
224	
225		# mkdir /new-root
226		# mount -o ro /dev/hda1 /new-root
227	
228	The root change is accomplished with the pivot_root system call, which
229	is also available via the ``pivot_root`` utility (see :manpage:`pivot_root(8)`
230	man page; ``pivot_root`` is distributed with util-linux version 2.10h or higher
231	[#f3]_). ``pivot_root`` moves the current root to a directory under the new
232	root, and puts the new root at its place. The directory for the old root
233	must exist before calling ``pivot_root``. Example::
234	
235		# cd /new-root
236		# mkdir initrd
237		# pivot_root . initrd
238	
239	Now, the init process may still access the old root via its
240	executable, shared libraries, standard input/output/error, and its
241	current root directory. All these references are dropped by the
242	following command::
243	
244		# exec chroot . what-follows <dev/console >dev/console 2>&1
245	
246	Where what-follows is a program under the new root, e.g. ``/sbin/init``
247	If the new root file system will be used with udev and has no valid
248	``/dev`` directory, udev must be initialized before invoking chroot in order
249	to provide ``/dev/console``.
250	
251	Note: implementation details of pivot_root may change with time. In order
252	to ensure compatibility, the following points should be observed:
253	
254	 - before calling pivot_root, the current directory of the invoking
255	   process should point to the new root directory
256	 - use . as the first argument, and the _relative_ path of the directory
257	   for the old root as the second argument
258	 - a chroot program must be available under the old and the new root
259	 - chroot to the new root afterwards
260	 - use relative paths for dev/console in the exec command
261	
262	Now, the initrd can be unmounted and the memory allocated by the RAM
263	disk can be freed::
264	
265		# umount /initrd
266		# blockdev --flushbufs /dev/ram0
267	
268	It is also possible to use initrd with an NFS-mounted root, see the
269	:manpage:`pivot_root(8)` man page for details.
270	
271	
272	Usage scenarios
273	---------------
274	
275	The main motivation for implementing initrd was to allow for modular
276	kernel configuration at system installation. The procedure would work
277	as follows:
278	
279	  1) system boots from floppy or other media with a minimal kernel
280	     (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and
281	     loads initrd
282	  2) ``/sbin/init`` determines what is needed to (1) mount the "real" root FS
283	     (i.e. device type, device drivers, file system) and (2) the
284	     distribution media (e.g. CD-ROM, network, tape, ...). This can be
285	     done by asking the user, by auto-probing, or by using a hybrid
286	     approach.
287	  3) ``/sbin/init`` loads the necessary kernel modules
288	  4) ``/sbin/init`` creates and populates the root file system (this doesn't
289	     have to be a very usable system yet)
290	  5) ``/sbin/init`` invokes ``pivot_root`` to change the root file system and
291	     execs - via chroot - a program that continues the installation
292	  6) the boot loader is installed
293	  7) the boot loader is configured to load an initrd with the set of
294	     modules that was used to bring up the system (e.g. ``/initrd`` can be
295	     modified, then unmounted, and finally, the image is written from
296	     ``/dev/ram0`` or ``/dev/rd/0`` to a file)
297	  8) now the system is bootable and additional installation tasks can be
298	     performed
299	
300	The key role of initrd here is to re-use the configuration data during
301	normal system operation without requiring the use of a bloated "generic"
302	kernel or re-compiling or re-linking the kernel.
303	
304	A second scenario is for installations where Linux runs on systems with
305	different hardware configurations in a single administrative domain. In
306	such cases, it is desirable to generate only a small set of kernels
307	(ideally only one) and to keep the system-specific part of configuration
308	information as small as possible. In this case, a common initrd could be
309	generated with all the necessary modules. Then, only ``/sbin/init`` or a file
310	read by it would have to be different.
311	
312	A third scenario is more convenient recovery disks, because information
313	like the location of the root FS partition doesn't have to be provided at
314	boot time, but the system loaded from initrd can invoke a user-friendly
315	dialog and it can also perform some sanity checks (or even some form of
316	auto-detection).
317	
318	Last not least, CD-ROM distributors may use it for better installation
319	from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
320	via initrd from CD; or by booting via a loader like ``LOADLIN`` or directly
321	from the CD-ROM, and loading the RAM disk from CD without need of
322	floppies.
323	
324	
325	Obsolete root change mechanism
326	------------------------------
327	
328	The following mechanism was used before the introduction of pivot_root.
329	Current kernels still support it, but you should _not_ rely on its
330	continued availability.
331	
332	It works by mounting the "real" root device (i.e. the one set with rdev
333	in the kernel image or with root=... at the boot command line) as the
334	root file system when linuxrc exits. The initrd file system is then
335	unmounted, or, if it is still busy, moved to a directory ``/initrd``, if
336	such a directory exists on the new root file system.
337	
338	In order to use this mechanism, you do not have to specify the boot
339	command options root, init, or rw. (If specified, they will affect
340	the real root file system, not the initrd environment.)
341	
342	If /proc is mounted, the "real" root device can be changed from within
343	linuxrc by writing the number of the new root FS device to the special
344	file /proc/sys/kernel/real-root-dev, e.g.::
345	
346	  # echo 0x301 >/proc/sys/kernel/real-root-dev
347	
348	Note that the mechanism is incompatible with NFS and similar file
349	systems.
350	
351	This old, deprecated mechanism is commonly called ``change_root``, while
352	the new, supported mechanism is called ``pivot_root``.
353	
354	
355	Mixed change_root and pivot_root mechanism
356	------------------------------------------
357	
358	In case you did not want to use ``root=/dev/ram0`` to trigger the pivot_root
359	mechanism, you may create both ``/linuxrc`` and ``/sbin/init`` in your initrd
360	image.
361	
362	``/linuxrc`` would contain only the following::
363	
364		#! /bin/sh
365		mount -n -t proc proc /proc
366		echo 0x0100 >/proc/sys/kernel/real-root-dev
367		umount -n /proc
368	
369	Once linuxrc exited, the kernel would mount again your initrd as root,
370	this time executing ``/sbin/init``. Again, it would be the duty of this init
371	to build the right environment (maybe using the ``root= device`` passed on
372	the cmdline) before the final execution of the real ``/sbin/init``.
373	
374	
375	Resources
376	---------
377	
378	.. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future"
379	    http://www.almesberger.net/cv/papers/ols2k-9.ps.gz
380	.. [#f2] newlib package (experimental), with initrd example
381	    https://www.sourceware.org/newlib/
382	.. [#f3] util-linux: Miscellaneous utilities for Linux
383	    https://www.kernel.org/pub/linux/utils/util-linux/
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