path: root/Documentation/initrd.txt
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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/initrd.txt
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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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].
+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) initrd is mounted read-write as root
+ 4) /linuxrc 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)
+ 5) linuxrc mounts the "real" root file system
+ 6) linuxrc places the root file system at the root directory using the
+ pivot_root system call
+ 7) the usual boot sequence (e.g. invocation of /sbin/init) is performed
+ on the root file system
+ 8) 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 (without devfs)
+ root=/dev/rd/0 (with devfs)
+ initrd is mounted as root, and the normal boot procedure is followed,
+ with the RAM disk still mounted as root.
+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 (note that this
+does not work if using devfs):
+# 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 (not necessary if using devfs, but it can't
+ hurt to do it anyway):
+ # 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, /linuxrc
+ Note that /linuxrc's permissions must include "x" (execute).
+ 6) correct operation the initrd environment can frequently be tested
+ even without rebooting with the command
+ # chroot /mnt /linuxrc
+ 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 /linuxrc to /bin/sh. Alternatively, you
+can try the experimental newlib environment [2] to create a small
+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 init=/linuxrc rw
+if not using devfs, or
+ root=/dev/rd/0 init=/linuxrc rw
+if using devfs. (rw is only necessary if writing to the initrd file
+With LOADLIN, you simply execute
+ LOADLIN <kernel> initrd=<disk_image>
+e.g. LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0
+ init=/linuxrc 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 init=/linuxrc 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, linuxrc 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 linuxrc 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 devfs and has no valid
+/dev directory, devfs must be mounted 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 # /dev/rd/0 if using devfs
+It is also possible to use initrd with an NFS-mounted root, see the
+pivot_root(8) man page for details.
+Note: if linuxrc or any program exec'ed from it terminates for some
+reason, the old change_root mechanism is invoked (see section "Obsolete
+root change mechanism").
+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) /linuxrc 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) /linuxrc loads the necessary kernel modules
+ 4) /linuxrc creates and populates the root file system (this doesn't
+ have to be a very usable system yet)
+ 5) /linuxrc 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 /linuxrc or a file
+read by it would have to be different.
+A third scenario are 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
+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
+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
+This old, deprecated mechanism is commonly called "change_root", while
+the new, supported mechanism is called "pivot_root".
+[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] Brouwer, Andries; "util-linux: Miscellaneous utilities for Linux"
+ ftp://ftp.win.tue.nl/pub/linux-local/utils/util-linux/

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