384 lines
14 KiB
ReStructuredText
384 lines
14 KiB
ReStructuredText
Using the initial RAM disk (initrd)
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===================================
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Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
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Hans Lermen <lermen@fgan.de>
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initrd provides the capability to load a RAM disk by the boot loader.
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This RAM disk can then be mounted as the root file system and programs
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can be run from it. Afterwards, a new root file system can be mounted
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from a different device. The previous root (from initrd) is then moved
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to a directory and can be subsequently unmounted.
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initrd is mainly designed to allow system startup to occur in two phases,
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where the kernel comes up with a minimum set of compiled-in drivers, and
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where additional modules are loaded from initrd.
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This document gives a brief overview of the use of initrd. A more detailed
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discussion of the boot process can be found in [#f1]_.
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Operation
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---------
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When using initrd, the system typically boots as follows:
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1) the boot loader loads the kernel and the initial RAM disk
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2) the kernel converts initrd into a "normal" RAM disk and
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frees the memory used by initrd
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3) if the root device is not ``/dev/ram0``, the old (deprecated)
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change_root procedure is followed. see the "Obsolete root change
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mechanism" section below.
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4) root device is mounted. if it is ``/dev/ram0``, the initrd image is
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then mounted as root
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5) /sbin/init is executed (this can be any valid executable, including
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shell scripts; it is run with uid 0 and can do basically everything
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init can do).
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6) init mounts the "real" root file system
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7) init places the root file system at the root directory using the
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pivot_root system call
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8) init execs the ``/sbin/init`` on the new root filesystem, performing
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the usual boot sequence
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9) the initrd file system is removed
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Note that changing the root directory does not involve unmounting it.
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It is therefore possible to leave processes running on initrd during that
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procedure. Also note that file systems mounted under initrd continue to
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be accessible.
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Boot command-line options
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-------------------------
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initrd adds the following new options::
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initrd=<path> (e.g. LOADLIN)
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Loads the specified file as the initial RAM disk. When using LILO, you
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have to specify the RAM disk image file in /etc/lilo.conf, using the
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INITRD configuration variable.
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noinitrd
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initrd data is preserved but it is not converted to a RAM disk and
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the "normal" root file system is mounted. initrd data can be read
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from /dev/initrd. Note that the data in initrd can have any structure
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in this case and doesn't necessarily have to be a file system image.
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This option is used mainly for debugging.
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Note: /dev/initrd is read-only and it can only be used once. As soon
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as the last process has closed it, all data is freed and /dev/initrd
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can't be opened anymore.
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root=/dev/ram0
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initrd is mounted as root, and the normal boot procedure is followed,
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with the RAM disk mounted as root.
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Compressed cpio images
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----------------------
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Recent kernels have support for populating a ramdisk from a compressed cpio
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archive. On such systems, the creation of a ramdisk image doesn't need to
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involve special block devices or loopbacks; you merely create a directory on
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disk with the desired initrd content, cd to that directory, and run (as an
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example)::
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find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
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Examining the contents of an existing image file is just as simple::
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mkdir /tmp/imagefile
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cd /tmp/imagefile
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gzip -cd /boot/imagefile.img | cpio -imd --quiet
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Installation
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------------
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First, a directory for the initrd file system has to be created on the
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"normal" root file system, e.g.::
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# mkdir /initrd
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The name is not relevant. More details can be found on the
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:manpage:`pivot_root(2)` man page.
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If the root file system is created during the boot procedure (i.e. if
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you're building an install floppy), the root file system creation
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procedure should create the ``/initrd`` directory.
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If initrd will not be mounted in some cases, its content is still
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accessible if the following device has been created::
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# mknod /dev/initrd b 1 250
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# chmod 400 /dev/initrd
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Second, the kernel has to be compiled with RAM disk support and with
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support for the initial RAM disk enabled. Also, at least all components
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needed to execute programs from initrd (e.g. executable format and file
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system) must be compiled into the kernel.
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Third, you have to create the RAM disk image. This is done by creating a
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file system on a block device, copying files to it as needed, and then
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copying the content of the block device to the initrd file. With recent
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kernels, at least three types of devices are suitable for that:
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- a floppy disk (works everywhere but it's painfully slow)
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- a RAM disk (fast, but allocates physical memory)
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- a loopback device (the most elegant solution)
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We'll describe the loopback device method:
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1) make sure loopback block devices are configured into the kernel
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2) create an empty file system of the appropriate size, e.g.::
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# dd if=/dev/zero of=initrd bs=300k count=1
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# mke2fs -F -m0 initrd
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(if space is critical, you may want to use the Minix FS instead of Ext2)
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3) mount the file system, e.g.::
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# mount -t ext2 -o loop initrd /mnt
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4) create the console device::
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# mkdir /mnt/dev
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# mknod /mnt/dev/console c 5 1
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5) copy all the files that are needed to properly use the initrd
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environment. Don't forget the most important file, ``/sbin/init``
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.. note:: ``/sbin/init`` permissions must include "x" (execute).
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6) correct operation the initrd environment can frequently be tested
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even without rebooting with the command::
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# chroot /mnt /sbin/init
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This is of course limited to initrds that do not interfere with the
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general system state (e.g. by reconfiguring network interfaces,
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overwriting mounted devices, trying to start already running demons,
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etc. Note however that it is usually possible to use pivot_root in
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such a chroot'ed initrd environment.)
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7) unmount the file system::
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# umount /mnt
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8) the initrd is now in the file "initrd". Optionally, it can now be
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compressed::
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# gzip -9 initrd
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For experimenting with initrd, you may want to take a rescue floppy and
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only add a symbolic link from ``/sbin/init`` to ``/bin/sh``. Alternatively, you
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can try the experimental newlib environment [#f2]_ to create a small
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initrd.
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Finally, you have to boot the kernel and load initrd. Almost all Linux
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boot loaders support initrd. Since the boot process is still compatible
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with an older mechanism, the following boot command line parameters
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have to be given::
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root=/dev/ram0 rw
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(rw is only necessary if writing to the initrd file system.)
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With LOADLIN, you simply execute::
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LOADLIN <kernel> initrd=<disk_image>
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e.g.::
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LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw
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With LILO, you add the option ``INITRD=<path>`` to either the global section
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or to the section of the respective kernel in ``/etc/lilo.conf``, and pass
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the options using APPEND, e.g.::
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image = /bzImage
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initrd = /boot/initrd.gz
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append = "root=/dev/ram0 rw"
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and run ``/sbin/lilo``
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For other boot loaders, please refer to the respective documentation.
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Now you can boot and enjoy using initrd.
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Changing the root device
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------------------------
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When finished with its duties, init typically changes the root device
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and proceeds with starting the Linux system on the "real" root device.
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The procedure involves the following steps:
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- mounting the new root file system
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- turning it into the root file system
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- removing all accesses to the old (initrd) root file system
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- unmounting the initrd file system and de-allocating the RAM disk
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Mounting the new root file system is easy: it just needs to be mounted on
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a directory under the current root. Example::
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# mkdir /new-root
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# mount -o ro /dev/hda1 /new-root
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The root change is accomplished with the pivot_root system call, which
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is also available via the ``pivot_root`` utility (see :manpage:`pivot_root(8)`
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man page; ``pivot_root`` is distributed with util-linux version 2.10h or higher
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[#f3]_). ``pivot_root`` moves the current root to a directory under the new
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root, and puts the new root at its place. The directory for the old root
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must exist before calling ``pivot_root``. Example::
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# cd /new-root
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# mkdir initrd
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# pivot_root . initrd
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Now, the init process may still access the old root via its
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executable, shared libraries, standard input/output/error, and its
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current root directory. All these references are dropped by the
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following command::
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# exec chroot . what-follows <dev/console >dev/console 2>&1
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Where what-follows is a program under the new root, e.g. ``/sbin/init``
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If the new root file system will be used with udev and has no valid
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``/dev`` directory, udev must be initialized before invoking chroot in order
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to provide ``/dev/console``.
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Note: implementation details of pivot_root may change with time. In order
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to ensure compatibility, the following points should be observed:
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- before calling pivot_root, the current directory of the invoking
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process should point to the new root directory
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- use . as the first argument, and the _relative_ path of the directory
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for the old root as the second argument
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- a chroot program must be available under the old and the new root
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- chroot to the new root afterwards
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- use relative paths for dev/console in the exec command
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Now, the initrd can be unmounted and the memory allocated by the RAM
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disk can be freed::
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# umount /initrd
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# blockdev --flushbufs /dev/ram0
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It is also possible to use initrd with an NFS-mounted root, see the
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:manpage:`pivot_root(8)` man page for details.
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Usage scenarios
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---------------
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The main motivation for implementing initrd was to allow for modular
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kernel configuration at system installation. The procedure would work
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as follows:
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1) system boots from floppy or other media with a minimal kernel
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(e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and
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loads initrd
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2) ``/sbin/init`` determines what is needed to (1) mount the "real" root FS
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(i.e. device type, device drivers, file system) and (2) the
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distribution media (e.g. CD-ROM, network, tape, ...). This can be
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done by asking the user, by auto-probing, or by using a hybrid
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approach.
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3) ``/sbin/init`` loads the necessary kernel modules
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4) ``/sbin/init`` creates and populates the root file system (this doesn't
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have to be a very usable system yet)
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5) ``/sbin/init`` invokes ``pivot_root`` to change the root file system and
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execs - via chroot - a program that continues the installation
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6) the boot loader is installed
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7) the boot loader is configured to load an initrd with the set of
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modules that was used to bring up the system (e.g. ``/initrd`` can be
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modified, then unmounted, and finally, the image is written from
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``/dev/ram0`` or ``/dev/rd/0`` to a file)
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8) now the system is bootable and additional installation tasks can be
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performed
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The key role of initrd here is to re-use the configuration data during
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normal system operation without requiring the use of a bloated "generic"
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kernel or re-compiling or re-linking the kernel.
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A second scenario is for installations where Linux runs on systems with
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different hardware configurations in a single administrative domain. In
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such cases, it is desirable to generate only a small set of kernels
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(ideally only one) and to keep the system-specific part of configuration
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information as small as possible. In this case, a common initrd could be
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generated with all the necessary modules. Then, only ``/sbin/init`` or a file
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read by it would have to be different.
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A third scenario is more convenient recovery disks, because information
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like the location of the root FS partition doesn't have to be provided at
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boot time, but the system loaded from initrd can invoke a user-friendly
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dialog and it can also perform some sanity checks (or even some form of
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auto-detection).
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Last not least, CD-ROM distributors may use it for better installation
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from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
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via initrd from CD; or by booting via a loader like ``LOADLIN`` or directly
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from the CD-ROM, and loading the RAM disk from CD without need of
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floppies.
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Obsolete root change mechanism
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------------------------------
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The following mechanism was used before the introduction of pivot_root.
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Current kernels still support it, but you should _not_ rely on its
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continued availability.
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It works by mounting the "real" root device (i.e. the one set with rdev
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in the kernel image or with root=... at the boot command line) as the
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root file system when linuxrc exits. The initrd file system is then
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unmounted, or, if it is still busy, moved to a directory ``/initrd``, if
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such a directory exists on the new root file system.
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In order to use this mechanism, you do not have to specify the boot
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command options root, init, or rw. (If specified, they will affect
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the real root file system, not the initrd environment.)
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If /proc is mounted, the "real" root device can be changed from within
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linuxrc by writing the number of the new root FS device to the special
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file /proc/sys/kernel/real-root-dev, e.g.::
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# echo 0x301 >/proc/sys/kernel/real-root-dev
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Note that the mechanism is incompatible with NFS and similar file
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systems.
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This old, deprecated mechanism is commonly called ``change_root``, while
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the new, supported mechanism is called ``pivot_root``.
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Mixed change_root and pivot_root mechanism
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------------------------------------------
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In case you did not want to use ``root=/dev/ram0`` to trigger the pivot_root
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mechanism, you may create both ``/linuxrc`` and ``/sbin/init`` in your initrd
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image.
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``/linuxrc`` would contain only the following::
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#! /bin/sh
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mount -n -t proc proc /proc
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echo 0x0100 >/proc/sys/kernel/real-root-dev
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umount -n /proc
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Once linuxrc exited, the kernel would mount again your initrd as root,
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this time executing ``/sbin/init``. Again, it would be the duty of this init
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to build the right environment (maybe using the ``root= device`` passed on
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the cmdline) before the final execution of the real ``/sbin/init``.
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Resources
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---------
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.. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future"
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http://www.almesberger.net/cv/papers/ols2k-9.ps.gz
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.. [#f2] newlib package (experimental), with initrd example
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https://www.sourceware.org/newlib/
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.. [#f3] util-linux: Miscellaneous utilities for Linux
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https://www.kernel.org/pub/linux/utils/util-linux/
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