Initrd

2011-01-27T12:20:04Z

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NAME
: initrd - boot loader initialized RAM disk

DESCRIPTION
: The special file /dev/initrd is a read-only block device.
: Device /dev/initrd is a RAM disk that is initialized (e.g.
: loaded) by the boot loader before the kernel is started.
: The kernel then can use the the block device /dev/initrd's
: contents for a two phased system boot-up.

: In the first boot-up phase, the kernel starts up and
: mounts an initial root file-system from the contents of
: /dev/initrd (e.g. RAM disk initialized by the boot
: loader). In the second phase, additional drivers or other
: modules are loaded from the initial root device's con-
: tents. After loading the additional modules, a new root
: file system (i.e. the normal root file system) is mounted
: from a different device.

BOOT-UP OPERATION
; When booting up with initrd, the system boots as follows : 1. The boot loader loads the kernel program and
: /dev/initrd's contents into memory.

: 2. On kernel startup, the kernel uncompresses and copies
: the contents of the device /dev/initrd onto device
: /dev/ram0 and then frees the memory used by /dev/initrd.

: 3. The kernel then read-write mounts device /dev/ram0 as
: the initial root file system.

: 4. If the indicated normal root file system is also the
: initial root file-system (e.g. /dev/ram0 ) then the
: kernel skips to the last step for the usual boot
: sequence.

: 5. If the executable file /linuxrc is present in the
: initial root file-system, /linuxrc is executed with uid
: 0. (The file /linuxrc must have executable permission.
: The file /linuxrc can be any valid executable, including
: a shell script.)

: 6. If /linuxrc is not executed or when /linuxrc termi-
: nates, the normal root file system is mounted. (If
: /linuxrc exits with any file-systems mounted on the ini-
: tial root file-system, then the behavior of the kernel
: is UNSPECIFIED. See the NOTES section for the current
: kernel behavior.)

: 7. If the normal root file has directory /initrd, device
: /dev/ram0 is moved from / to /initrd. Otherwise if
: directory /initrd does not exist device /dev/ram0 is
: unmounted. (When moved from / to /initrd, /dev/ram0 is
: not unmounted and therefore processes can remain running
: from /dev/ram0. If directory /initrd does not exist on
: the normal root file-system and any processes remain
: running from /dev/ram0 when /linuxrc exits, the behavior
: of the kernel is UNSPECIFIED. See the NOTES section for
: the current kernel behavior.)

: 8. The usual boot sequence (e.g. invocation of
: /sbin/init) is performed on the normal root file system.

OPTIONS
: The following boot loader options when used with initrd,
; affect the kernel's boot-up operation : initrd=filename
: Specifies the file to load as the contents of
: /dev/initrd. For LOADLIN this is a command line
: option. For LILO you have to use this command in
: the LILO configuration file /etc/lilo.config. The
: filename specified with this option will typically
: be a gzipped file-system image.

: noinitrd
: This boot time option disables the two phase boot-
: up operation. The kernel performs the usual boot
: sequence as if /dev/initrd was not initialized.
: With this option, any contents of /dev/initrd
: loaded into memory by the boot loader contents are
: preserved. This option permits the contents of
: /dev/initrd to be any data and need not be limited
: to a file system image. However, device /dev/ini-
: trd is read-only and can be read only one time
: after system startup.

: root=device-name
: Specifies the device to be used as the normal root
: file system. For LOADLIN this is a command line
: option. For LILO this is a boot time option or can
: be used as an option line in the LILO configuration
: file /etc/lilo.config. The device specified by the
: this option must be a mountable device having a
: suitable root file-system.

CHANGING THE NORMAL ROOT FILE SYSTEM
: By default, the kernel's settings (e.g. set in the kernel
: file with rdev or compiled into the kernel file), or the
: boot loader option setting is used for the normal root
: file systems. For a NFS-mounted normal root file system,
: one has to use the nfs''root''name and nfs''root''addrs boot
: options to give the NFS settings. For more information on
: NFS-mounted root see the kernel documentation file nfs-
: root.txt. For more information on setting the root file
: system also see the LILO and LOADLIN documentation.

: It is also possible for the /linuxrc executable to change
: the normal root device. For /linuxrc to change the normal
: root device, /proc must be mounted. After mounting /proc,
: /linuxrc changes the normal root device by writing into
: the proc files /proc/sys/kernel/real-root-dev,
: /proc/sys/kernel/nfs-root-name, and /proc/sys/kernel/nfs-
: root-addrs. For a physical root device, the root device
: is changed by having /linuxrc write the new root file sys-
: tem device number into /proc/sys/kernel/real-root-dev.
: For a NFS root file system, the root device is changed by
: having /linuxrc write the NFS setting into files
: /proc/sys/kernel/nfs-root-name and /proc/sys/kernel/nfs-
: root-addrs and then writing 0xff (e.g. the pseudo-NFS-
: device number) into file /proc/sys/kernel/real-root-dev.
: For example, the following shell command line would change
; the normal root device to /dev/hdb1 : echo 0x365 >/proc/sys/kernel/real-root-dev
: For a NFS example, the following shell command lines would
: change the normal root device to the NFS directory
: /var/nfsroot on a local networked NFS server with IP num-
: ber 193.8.232.7 for a system with IP number 193.8.232.7
; and named 'idefix' : echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
: echo 193.8.232.2:193.8.232.7::255.255.255.0:idefix \
: >/proc/sys/kernel/nfs-root-addrs
: echo 255 >/proc/sys/kernel/real-root-dev

USAGE
: The main motivation for implementing initrd was to allow
: for modular kernel configuration at system installation.

; A possible system installation scenario is as follows : 1. The loader program boots from floppy or other media
: with a minimal kernel (e.g. support for /dev/ram,
: /dev/initrd, and the ext2 file-system) and loads
: /dev/initrd with a gzipped version of the initial file-
: system.

: 2. The executable /linuxrc determines what is needed to
: (1) mount the normal root file-system (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. The executable /linuxrc loads the necessary modules
: from the initial root file-system.

: 4. The executable /linuxrc creates and populates the
: root file system. (At this stage the normal root file
: system does not have to be a completed system yet.)

: 5. The executable /linuxrc sets /proc/sys/kernel/real-
: root-dev, unmount /proc, the normal root file system and
: any other file systems it has mounted, and then termi-
: nates.

: 6. The kernel then mounts the normal root file system.

: 7. Now that the file system is accessible and intact,
: the boot loader can be installed.

: 8. The boot loader is configured to load into /dev/ini-
: trd a file system with the set of modules that was used
: to bring up the system. (e.g. Device /dev/ram0 can be
: modified, then unmounted, and finally, the image is
: written from /dev/ram0 to a file.)

: 9. The system is now bootable and additional installa-
: tion tasks can be performed.

: The key role of /dev/initrd in the above is to re-use the
: configuration data during normal system operation without
: requiring initial kernel selection, a large generic kernel
: or, recompiling the kernel.

: A second scenario is for installations where Linux runs on
: systems with different hardware configurations in a single
: administrative network. In such cases, it may be desir-
: able to use 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, create a
: common file with all needed modules. Then, only the the
: /linuxrc file or a file executed by /linuxrc would be dif-
: ferent.

: A third scenario is more convenient recovery disks.
: Because information like the location of the root file-
: system partition is not needed at boot time, the system
: loaded from /dev/initrd can use a dialog and/or auto-
: detection followed by a possible sanity check.

: Last but not least, Linux distributions on CD-ROM may use
: initrd for easy installation from the CD-ROM. The distri-
: bution can use LOADLIN to directly load /dev/initrd from
: CD-ROM without the need of any floppies. The distribution
: could also use a LILO boot floppy and then bootstrap a
: bigger ram disk via /dev/initrd from the CD-ROM.

CONFIGURATION
: The /dev/initrd is a read-only block device assigned major
: number 1 and minor number 250. Typically /dev/initrd is
: owned by root.disk with mode 0400 (read access by root
: only). If the Linux system does not have /dev/initrd
: already created, it can be created with the following com-
; mands : mknod -m 400 /dev/initrd b 1 250
: chown root:disk /dev/initrd

: Also, support for both "RAM disk" and "Initial RAM disk"
: (e.g. CONFIG''BLK''DEV''RAM=y and CONFIG''BLK''DEV''INITRD=y )
: support must be compiled directly into the Linux kernel to
: use /dev/initrd. When using /dev/initrd, the RAM disk
: driver cannot be loaded as a module.

FILES
: /dev/initrd
: /dev/ram0
: /linuxrc
: /initrd

SEE ALSO
: chown(1), mknod(1), /dev/ram(4), freeramdisk(8), rdev(8),
: The documentation file initrd.txt in the kernel source
: package, the LILO documentation, the LOADLIN documenta-
: tion, the SYSLINUX documentation.

NOTES
: 1. With the current kernel, any file systems that remain
: mounted when /dev/ram0 is moved from / to /initrd continue
: to be accessible. However, the /proc/mounts entries are
: not updated.

: 2. With the current kernel, if directory /initrd does not
: exist, then /dev/ram0 will NOT be fully unmounted if
: /dev/ram0 is used by any process or has any file-system
: mounted on it. If /dev/ram0 is NOT fully unmounted, then
: /dev/ram0 will remain in memory.

: 3. Users of /dev/initrd should not depend on the behavior
: give in the above notes. The behavior may change in
: future versions of the Linux kernel.

AUTHOR
: The kernel code for device initrd was written by Werner
: Almesberger <almesber@lrc.epfl.ch> and Hans Lermen <ler-
: men@elserv.ffm.fgan.de>. The code for initrd was added to
: the baseline Linux kernel in development version 1.3.73.
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Initrd