1 Using the initial RAM disk (initrd)
2 ===================================
4 Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
5 Hans Lermen <lermen@fgan.de>
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.
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.
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]_.
25 When using initrd, the system typically boots as follows:
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
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
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
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
51 Boot command-line options
52 -------------------------
54 initrd adds the following new options::
56 initrd=<path> (e.g. LOADLIN)
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.
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.
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.
76 initrd is mounted as root, and the normal boot procedure is followed,
77 with the RAM disk mounted as root.
79 Compressed cpio images
80 ----------------------
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
88 find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
90 Examining the contents of an existing image file is just as simple::
94 gzip -cd /boot/imagefile.img | cpio -imd --quiet
99 First, a directory for the initrd file system has to be created on the
100 "normal" root file system, e.g.::
104 The name is not relevant. More details can be found on the
105 :manpage:`pivot_root(2)` man page.
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.
111 If initrd will not be mounted in some cases, its content is still
112 accessible if the following device has been created::
114 # mknod /dev/initrd b 1 250
115 # chmod 400 /dev/initrd
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.
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:
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)
131 We'll describe the loopback device method:
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.::
136 # dd if=/dev/zero of=initrd bs=300k count=1
137 # mke2fs -F -m0 initrd
139 (if space is critical, you may want to use the Minix FS instead of Ext2)
140 3) mount the file system, e.g.::
142 # mount -t ext2 -o loop initrd /mnt
144 4) create the console device::
147 # mknod /mnt/dev/console c 5 1
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``
152 .. note:: ``/sbin/init`` permissions must include "x" (execute).
154 6) correct operation the initrd environment can frequently be tested
155 even without rebooting with the command::
157 # chroot /mnt /sbin/init
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::
168 8) the initrd is now in the file "initrd". Optionally, it can now be
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
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
185 (rw is only necessary if writing to the initrd file system.)
187 With LOADLIN, you simply execute::
189 LOADLIN <kernel> initrd=<disk_image>
193 LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw
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.::
200 initrd = /boot/initrd.gz
201 append = "root=/dev/ram0 rw"
203 and run ``/sbin/lilo``
205 For other boot loaders, please refer to the respective documentation.
207 Now you can boot and enjoy using initrd.
210 Changing the root device
211 ------------------------
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.
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
222 Mounting the new root file system is easy: it just needs to be mounted on
223 a directory under the current root. Example::
226 # mount -o ro /dev/hda1 /new-root
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::
237 # pivot_root . initrd
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
244 # exec chroot . what-follows <dev/console >dev/console 2>&1
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``.
251 Note: implementation details of pivot_root may change with time. In order
252 to ensure compatibility, the following points should be observed:
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
262 Now, the initrd can be unmounted and the memory allocated by the RAM
266 # blockdev --flushbufs /dev/ram0
268 It is also possible to use initrd with an NFS-mounted root, see the
269 :manpage:`pivot_root(8)` man page for details.
275 The main motivation for implementing initrd was to allow for modular
276 kernel configuration at system installation. The procedure would work
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
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
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
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.
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.
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
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
325 Obsolete root change mechanism
326 ------------------------------
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.
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.
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.)
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.::
346 # echo 0x301 >/proc/sys/kernel/real-root-dev
348 Note that the mechanism is incompatible with NFS and similar file
351 This old, deprecated mechanism is commonly called ``change_root``, while
352 the new, supported mechanism is called ``pivot_root``.
355 Mixed change_root and pivot_root mechanism
356 ------------------------------------------
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
362 ``/linuxrc`` would contain only the following::
365 mount -n -t proc proc /proc
366 echo 0x0100 >/proc/sys/kernel/real-root-dev
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``.
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/