Autodetection

2008-07-15T05:52:13Z

87.234.234.66: rolnoaceldro

racdron
[[autodetection]] is a system that probes the computer hardware and automatically determines what kinds of hardware are available on the motherboard or installed as an add in card in the computer chasis. Part of the [[autodetection]] process also configures the [[kernel]] to support the discovered hardware.

Initrd

2008-07-14T15:11:01Z

87.234.234.66: ractro

relacelcnal
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.

[[Category:Booting]]

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Autodetection

Initrd