Documentation/admin-guide/initrd.rst

15 where the kernel comes up with a minimum set of compiled-in drivers, and
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]_.
23 ---------
51 Boot command-line options
52 -------------------------
70     Note: /dev/initrd is read-only and it can only be used once. As soon
80 ----------------------
83 archive. On such systems, the creation of a ramdisk image doesn't need to
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
97 ------------
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)
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)
142 	# mount -t ext2 -o loop initrd /mnt
155     even without rebooting with the command::
159     This is of course limited to initrds that do not interfere with the
171 	# gzip -9 initrd
196 or to the section of the respective kernel in ``/etc/lilo.conf``, and pass
211 ------------------------
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
225 	# mkdir /new-root
226 	# mount -o ro /dev/hda1 /new-root
230 man page; ``pivot_root`` is distributed with util-linux version 2.10h or higher
235 	# cd /new-root
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``
251 Note: implementation details of pivot_root may change with time. In order
254  - before calling pivot_root, the current directory of the invoking
256  - use . as the first argument, and the _relative_ path of the directory
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
266 	# blockdev --flushbufs /dev/ram0
268 It is also possible to use initrd with an NFS-mounted root, see the
273 ---------------
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
291      execs - via chroot - a program that continues the installation
293   7) the boot loader is configured to load an initrd with the set of
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.
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
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).
318 Last not least, CD-ROM distributors may use it for better installation
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
326 ------------------------------
328 The following mechanism was used before the introduction of pivot_root.
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
356 ------------------------------------------
365 	mount -n -t proc proc /proc
366 	echo 0x0100 >/proc/sys/kernel/real-root-dev
367 	umount -n /proc
370 this time executing ``/sbin/init``. Again, it would be the duty of this init
372 the cmdline) before the final execution of the real ``/sbin/init``.
376 ---------
378 .. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future"
379     https://www.almesberger.net/cv/papers/ols2k-9.ps.gz
382 .. [#f3] util-linux: Miscellaneous utilities for Linux
383     https://www.kernel.org/pub/linux/utils/util-linux/