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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 [1].
-
-
-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 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 that /sbin/init's 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 [2] 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 pivot_root(8) man
-page; pivot_root is distributed with util-linux version 2.10h or higher
-[3]). 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
-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
----------
-
-[1] Almesberger, Werner; "Booting Linux: The History and the Future"
- http://www.almesberger.net/cv/papers/ols2k-9.ps.gz
-[2] newlib package (experimental), with initrd example
- http://sources.redhat.com/newlib/
-[3] util-linux: Miscellaneous utilities for Linux
- http://www.kernel.org/pub/linux/utils/util-linux/