mirror of https://github.com/mkerrisk/man-pages
484 lines
13 KiB
Groff
484 lines
13 KiB
Groff
.\" This man-page is Copyright (C) 1997 John S. Kallal
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and author(s) of this work.
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.\" %%%LICENSE_END
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.\"
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.\" If the you wish to distribute versions of this work under other
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.\" conditions than the above, please contact the author(s) at the following
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.\" for permission:
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.\"
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.\" John S. Kallal -
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.\" email: <kallal@voicenet.com>
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.\" mail: 518 Kerfoot Farm RD, Wilmington, DE 19803-2444, USA
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.\" phone: (302)654-5478
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.\"
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.\" $Id: initrd.4,v 0.9 1997/11/07 05:05:32 kallal Exp kallal $
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.TH INITRD 4 2010-09-04 "Linux" "Linux Programmer's Manual"
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.SH NAME
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initrd \- boot loader initialized RAM disk
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.SH CONFIGURATION
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.I /dev/initrd
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is a read-only block device assigned
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major number 1 and minor number 250.
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Typically
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.I /dev/initrd
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is owned by
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.I root.disk
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with mode 0400 (read access by root only).
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If the Linux system does not have
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.I /dev/initrd
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already created, it can be created with the following commands:
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.nf
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\fB
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mknod \-m 400 /dev/initrd b 1 250
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chown root:disk /dev/initrd
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\fP
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.fi
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.PP
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Also, support for both "RAM disk" and "Initial RAM disk"
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(e.g.,
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.BR CONFIG_BLK_DEV_RAM=y
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and
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.BR CONFIG_BLK_DEV_INITRD=y )
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must be compiled directly into the Linux kernel to use
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.IR /dev/initrd .
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When using
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.IR /dev/initrd ,
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the RAM disk driver cannot be loaded as a module.
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.\"
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.\"
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.\"
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.SH DESCRIPTION
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The special file
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.I /dev/initrd
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is a read-only block device.
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This device is a RAM disk that is initialized (e.g., loaded)
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by the boot loader before the kernel is started.
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The kernel then can use
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.IR /dev/initrd "'s "
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contents for a two-phase system boot-up.
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.PP
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In the first boot-up phase, the kernel starts up
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and mounts an initial root filesystem from the contents of
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.I /dev/initrd
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(e.g., RAM disk initialized by the boot loader).
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In the second phase, additional drivers or other modules
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are loaded from the initial root device's contents.
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After loading the additional modules, a new root filesystem
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(i.e., the normal root filesystem) is mounted from a
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different device.
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.\"
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.\"
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.\"
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.SS Boot-up operation
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When booting up with
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.BR initrd ,
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the system boots as follows:
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.IP 1. 3
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The boot loader loads the kernel program and
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.IR /dev/initrd 's
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contents into memory.
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.IP 2.
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On kernel startup,
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the kernel uncompresses and copies the contents of the device
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.I /dev/initrd
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onto device
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.I /dev/ram0
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and then frees the memory used by
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.IR /dev/initrd .
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.IP 3.
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The kernel then read-write mounts the device
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.I /dev/ram0
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as the initial root filesystem.
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.IP 4.
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If the indicated normal root filesystem is also the initial
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root filesystem (e.g.,
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.IR /dev/ram0 )
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then the kernel skips to the last step for the usual boot sequence.
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.IP 5.
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If the executable file
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.IR /linuxrc
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is present in the initial root filesystem,
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.I /linuxrc
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is executed with UID 0.
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(The file
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.I /linuxrc
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must have executable permission.
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The file
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.I /linuxrc
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can be any valid executable, including a shell script.)
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.IP 6.
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If
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.I /linuxrc
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is not executed or when
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.I /linuxrc
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terminates, the normal root filesystem is mounted.
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(If
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.I /linuxrc
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exits with any filesystems mounted on the initial root
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filesystem, then the behavior of the kernel is
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.BR UNSPECIFIED .
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See the NOTES section for the current kernel behavior.)
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.IP 7.
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If the normal root filesystem has a directory
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.IR /initrd ,
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the device
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.I /dev/ram0
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is moved from
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.IR /
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to
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.IR /initrd .
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Otherwise, if the directory
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.IR /initrd
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does not exist, the device
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.I /dev/ram0
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is unmounted.
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(When moved from
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.IR /
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to
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.IR /initrd ,
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.I /dev/ram0
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is not unmounted and therefore processes can remain running from
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.IR /dev/ram0 .
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If directory
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.I /initrd
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does not exist on the normal root filesystem
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and any processes remain running from
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.IR /dev/ram0
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when
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.I /linuxrc
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exits, the behavior of the kernel is
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.BR UNSPECIFIED .
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See the NOTES section for the current kernel behavior.)
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.IP 8.
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The usual boot sequence (e.g., invocation of
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.IR /sbin/init )
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is performed on the normal root filesystem.
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.\"
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.\"
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.\"
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.SS Options
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The following boot loader options, when used with
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.BR initrd ,
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affect the kernel's boot-up operation:
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.TP
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.BI initrd= "filename"
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Specifies the file to load as the contents of
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.IR /dev/initrd .
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For
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.B LOADLIN
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this is a command-line option.
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For
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.B LILO
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you have to use this command in the
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.B LILO
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configuration file
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.IR /etc/lilo.config .
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The filename specified with this
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option will typically be a gzipped filesystem image.
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.TP
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.I noinitrd
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This boot option disables the two-phase boot-up operation.
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The kernel performs the usual boot sequence as if
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.I /dev/initrd
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was not initialized.
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With this option, any contents of
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.I /dev/initrd
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loaded into memory by the boot loader contents are preserved.
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This option permits the contents of
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.I /dev/initrd
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to be any data and need not be limited to a filesystem image.
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However, device
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.I /dev/initrd
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is read-only and can be read only one time after system startup.
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.TP
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.BI root= "device-name"
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Specifies the device to be used as the normal root filesystem.
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For
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.B LOADLIN
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this is a command-line option.
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For
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.B LILO
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this is a boot time option or
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can be used as an option line in the
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.B LILO
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configuration file
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.IR /etc/lilo.config .
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The device specified by the this option must be a mountable
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device having a suitable root filesystem.
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.\"
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.\"
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.\"
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.SS Changing the normal root filesystem
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By default,
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the kernel's settings
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(e.g., set in the kernel file with
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.BR rdev (8)
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or compiled into the kernel file),
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or the boot loader option setting
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is used for the normal root filesystems.
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For an NFS-mounted normal root filesystem, one has to use the
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.B nfs_root_name
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and
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.B nfs_root_addrs
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boot options to give the NFS settings.
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For more information on NFS-mounted root see the kernel documentation file
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.BR Documentation/filesystems/nfsroot.txt .
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For more information on setting the root filesystem see also the
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.BR LILO
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and
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.BR LOADLIN
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documentation.
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.PP
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It is also possible for the
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.I /linuxrc
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executable to change the normal root device.
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For
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.I /linuxrc
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to change the normal root device,
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.IR /proc
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must be mounted.
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After mounting
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.IR /proc ,
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.I /linuxrc
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changes the normal root device by writing into the proc files
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.IR /proc/sys/kernel/real-root-dev ,
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.IR /proc/sys/kernel/nfs-root-name ,
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and
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.IR /proc/sys/kernel/nfs-root-addrs .
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For a physical root device, the root device is changed by having
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.I /linuxrc
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write the new root filesystem device number into
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.IR /proc/sys/kernel/real-root-dev .
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For an NFS root filesystem, the root device is changed by having
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.I /linuxrc
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write the NFS setting into files
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.IR /proc/sys/kernel/nfs-root-name
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and
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.I /proc/sys/kernel/nfs-root-addrs
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and then writing 0xff (e.g., the pseudo-NFS-device number) into file
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.IR /proc/sys/kernel/real-root-dev .
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For example, the following shell command line would change
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the normal root device to
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.IR /dev/hdb1 :
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.nf
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echo 0x365 >/proc/sys/kernel/real-root-dev
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.fi
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For an NFS example, the following shell command lines would change the
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normal root device to the NFS directory
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.I /var/nfsroot
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on a local networked NFS server with IP number 193.8.232.7 for a system with
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IP number 193.8.232.2 and named "idefix":
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.nf
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echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
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echo 193.8.232.2:193.8.232.7::255.255.255.0:idefix \\
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>/proc/sys/kernel/nfs-root-addrs
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echo 255 >/proc/sys/kernel/real-root-dev
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.fi
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.BR Note :
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The use of
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.I /proc/sys/kernel/real-root-dev
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to change the root filesystem is obsolete.
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See the Linux kernel source file
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.I Documentation/initrd.txt
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as well as
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.BR pivot_root (2)
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and
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.BR pivot_root (8)
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for information on the modern method of changing the root filesystem.
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.\" FIXME . Should this manual page describe the pivot_root mechanism?
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.\"
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.\"
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.\"
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.SS Usage
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The main motivation for implementing
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.B initrd
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was to allow for modular kernel configuration at system installation.
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.PP
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A possible system installation scenario is as follows:
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.IP 1. 3
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The loader program boots from floppy or other media with a minimal kernel
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(e.g., support for
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.IR /dev/ram ,
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.IR /dev/initrd ,
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and the ext2 filesystem) and loads
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.IR /dev/initrd
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with a gzipped version of the initial filesystem.
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.IP 2.
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The executable
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.I /linuxrc
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determines what is needed to (1) mount the normal root filesystem
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(i.e., device type, device drivers, filesystem) and (2) the
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distribution media (e.g., CD-ROM, network, tape, ...).
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This can be done by asking the user, by auto-probing,
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or by using a hybrid approach.
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.IP 3.
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The executable
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.I /linuxrc
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loads the necessary modules from the initial root filesystem.
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.IP 4.
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The executable
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.I /linuxrc
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creates and populates the root filesystem.
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(At this stage the normal root filesystem does not have to be a
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completed system yet.)
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.IP 5.
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The executable
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.IR /linuxrc
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sets
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.IR /proc/sys/kernel/real-root-dev ,
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unmount
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.IR /proc ,
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the normal root filesystem and any other filesystems
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it has mounted, and then terminates.
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.IP 6.
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The kernel then mounts the normal root filesystem.
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.IP 7.
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Now that the filesystem is accessible and intact,
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the boot loader can be installed.
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.IP 8.
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The boot loader is configured to load into
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.I /dev/initrd
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a filesystem with the set of modules that was used to bring up the system.
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(e.g., Device
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.I /dev/ram0
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can be modified, then unmounted, and finally, the image is written from
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.I /dev/ram0
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to a file.)
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.IP 9.
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The system is now bootable and additional installation tasks can be
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performed.
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.PP
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The key role of
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.I /dev/initrd
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in the above is to reuse the configuration data during normal system operation
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without requiring initial kernel selection, a large generic kernel or,
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recompiling the kernel.
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.PP
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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 network.
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In such cases, it may be desirable to use 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.
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In this case, create a common file
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with all needed modules.
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Then, only the
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.I /linuxrc
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file or a file executed by
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.I /linuxrc
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would be different.
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.PP
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A third scenario is more convenient recovery disks.
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Because information like the location of the root filesystem
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partition is not needed at boot time, the system loaded from
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.I /dev/initrd
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can use a dialog and/or auto-detection followed by a
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possible sanity check.
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.PP
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Last but not least, Linux distributions on CD-ROM may use
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.B initrd
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for easy installation from the CD-ROM.
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The distribution can use
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.B LOADLIN
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to directly load
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.I /dev/initrd
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from CD-ROM without the need of any floppies.
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The distribution could also use a
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.B LILO
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boot floppy and then bootstrap a bigger RAM disk via
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.IR /dev/initrd
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from the CD-ROM.
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.\"
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.\"
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.\"
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.SH FILES
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.I /dev/initrd
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.br
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.I /dev/ram0
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.br
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.I /linuxrc
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.br
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.I /initrd
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.\"
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.\"
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.\"
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.SH NOTES
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.IP 1. 3
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With the current kernel, any filesystems that remain mounted when
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.I /dev/ram0
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is moved from
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.I /
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to
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.I /initrd
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continue to be accessible.
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However, the
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.I /proc/mounts
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entries are not updated.
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.IP 2.
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With the current kernel, if directory
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.I /initrd
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does not exist, then
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.I /dev/ram0
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will
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.B not
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be fully unmounted if
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.I /dev/ram0
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is used by any process or has any filesystem mounted on it.
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If
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.IR /dev/ram0
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is
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.B not
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fully unmounted, then
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.I /dev/ram0
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will remain in memory.
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.IP 3.
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Users of
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.I /dev/initrd
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should not depend on the behavior give in the above notes.
|
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The behavior may change in future versions of the Linux kernel.
|
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.\"
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.\"
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.\"
|
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.\" .SH AUTHORS
|
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.\" The kernel code for device
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.\" .BR initrd
|
|
.\" was written by Werner Almesberger <almesber@lrc.epfl.ch> and
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.\" Hans Lermen <lermen@elserv.ffm.fgan.de>.
|
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.\" The code for
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|
.\" .BR initrd
|
|
.\" was added to the baseline Linux kernel in development version 1.3.73.
|
|
.SH SEE ALSO
|
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.BR chown (1),
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.BR mknod (1),
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.BR ram (4),
|
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.BR freeramdisk (8),
|
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.BR rdev (8)
|
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|
|
.I Documentation/initrd.txt
|
|
in the Linux kernel source tree, the LILO documentation,
|
|
the LOADLIN documentation, the SYSLINUX documentation
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