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diff --git a/man4/initrd.4 b/man4/initrd.4 new file mode 100644 index 000000000..bc1d3b069 --- /dev/null +++ b/man4/initrd.4 @@ -0,0 +1,422 @@ +.\" -*- nroff -*- +.\" This man-page is Copyright (C) 1997 John S. Kallal +.\" +.\" Permission is granted to make and distribute verbatim copies of this +.\" manual provided the copyright notice and this permission notice are +.\" preserved on all copies. +.\" +.\" Permission is granted to copy and distribute modified versions of this +.\" manual under the conditions for verbatim copying, provided that the +.\" entire resulting derived work is distributed under the terms of a +.\" permission notice identical to this one. +.\" +.\" Since the Linux kernel and libraries are constantly changing, this +.\" manual page may be incorrect or out-of-date. The author(s) assume no +.\" responsibility for errors or omissions, or for damages resulting from +.\" the use of the information contained herein. The author(s) may not +.\" have taken the same level of care in the production of this manual, +.\" which is licensed free of charge, as they might when working +.\" professionally. +.\" +.\" Formatted or processed versions of this manual, if unaccompanied by +.\" the source, must acknowledge the copyright and author(s) of this work. +.\" +.\" If the you wish to distribute versions of this work under other +.\" conditions than the above, please contact the author(s) at the following +.\" for permission: +.\" +.\" John S. Kallal - +.\" email: <kallal@voicenet.com> +.\" mail: 518 Kerfoot Farm RD, Wilmington, DE 19803-2444, USA +.\" phone: (302)654-5478 +.\" +.\" $Id: initrd.4,v 0.9 1997/11/07 05:05:32 kallal Exp kallal $ +.TH INITRD 4 1997-11-06 "Linux 2.0" "Linux Programmer's Manual" +.SH NAME +initrd \- boot loader initialized RAM disk +.\" +.\" +.\" +.SH DESCRIPTION +The special file +.B /dev/initrd +is a read-only block device. +Device +.B /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 +.BR /dev/initrd "'s " +contents for a two phased system boot-up. +.PP +In the first boot-up phase, the kernel starts up +and mounts an initial root file-system from the contents +of +.B /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 contents. +After loading the additional modules, a new root file system +(i.e. the normal root file system) is mounted from a +different device. +.\" +.\" +.\" +.SH "BOOT-UP OPERATION" +When booting up with +.BR initrd ", the system boots as follows:" +.RS 0.2i +.PP +1. The boot loader loads the kernel program and +.BR /dev/initrd "'s contents into memory." +.PP +2. On kernel startup, the kernel uncompresses and copies the contents of the device +.B /dev/initrd +onto device +.B /dev/ram0 +and then frees the memory used by +.BR /dev/initrd "." +.PP +3. The kernel then read-write mounts device +.B /dev/ram0 +as the initial root file system. +.PP +4. If the indicated normal root file system is also the initial root file-system +(e.g. +.B /dev/ram0 +) then the kernel skips to the last step for the usual boot sequence. +.PP +5. If the executable file +.BR /linuxrc " is present in the initial root file-system, " /linuxrc +is executed with uid 0. +(The file +.B /linuxrc +must have executable permission. +The file +.B /linuxrc +can be any valid executable, including a shell script.) +.PP +6. If +.B /linuxrc +is not executed or when +.B /linuxrc +terminates, the normal root file system is mounted. +(If +.BR /linuxrc +exits with any file-systems mounted on the initial root +file-system, then the behavior of the kernel is +.BR UNSPECIFIED "." +See the +.BR NOTES +section for the current kernel behavior.) +.PP +7. If the normal root file has directory +.BR /initrd ", device" +.B /dev/ram0 +is moved from +.BR / " to " /initrd "." +Otherwise if directory +.BR /initrd " does not exist device " /dev/ram0 " is unmounted." +(When moved from +.BR / " to " /initrd ", " /dev/ram0 +is not unmounted and therefore processes can remain running from +.BR /dev/ram0 "." +If directory +.BR /initrd +does not exist on the normal root file-system +and any processes remain running from +.BR /dev/ram0 " when " /linuxrc +exits, the behavior of the kernel is +.BR UNSPECIFIED "." +See the +.BR NOTES +section for the current kernel behavior.) +.PP +8. The usual boot sequence (e.g. invocation of +.BR /sbin/init ") is performed on the normal root file system." +.\" +.\" +.\" +.SH OPTIONS +The following boot loader options when used with +.BR initrd ", affect the kernel's boot-up operation:" +.TP +.BI initrd= "filename" +Specifies the file to load as the contents of +.BR /dev/initrd "." +.RB "For " LOADLIN +this is a command line option. +.RB "For " LILO " you have to use this command in the +.BR LILO " configuration file " /etc/lilo.config "." +The filename specified with this +option will typically be a gzipped file-system image. +.TP +.B noinitrd +This boot time option disables the two phase boot-up operation. +The kernel performs the usual boot sequence as if +.B /dev/initrd +was not initialized. +With this option, any contents of +.B /dev/initrd +loaded into memory by the boot loader contents are preserved. +This option permits the contents of +.B /dev/initrd +to be any data and need not be limited to a file system image. +However, device +.B /dev/initrd +is read-only and can be read only one time after system startup. +.TP +.BI root= "device-name" +Specifies the device to be used as the normal root file system. +.RB "For " LOADLIN +this is a command line option. +.RB "For " LILO " this is a boot time option or +can be used as an option line in the +.BR LILO " configuration file " /etc/lilo.config "." +The device specified by the this option must be a mountable +device having a suitable root file-system. +.\" +.\" +.\" +.SH "CHANGING THE NORMAL ROOT FILE SYSTEM" +By default, +the kernel's settings +(e.g. set in the kernel file with +.B 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 +.BR 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 +.BR nfsroot.txt "." +For more information on setting the root file system also see the +.BR LILO " and " LOADLIN " documentation." +.PP +It is also possible for the +.BR /linuxrc +executable to change the normal root device. +For +.BR /linuxrc +to change the normal root device, +.BR /proc " must be mounted." +After mounting +.BR /proc ", " /linuxrc +changes the normal root device by writing into the proc files +.BR /proc/sys/kernel/real-root-dev ", " +.BR /proc/sys/kernel/nfs-root-name ", and " +.BR /proc/sys/kernel/nfs-root-addrs "." +For a physical root device, the root device is changed by having +.BR /linuxrc +write the new root file system device number into +.BR /proc/sys/kernel/real-root-dev "." +For a NFS root file system, the root device is changed by having +.BR /linuxrc +write the NFS setting into files +.BR /proc/sys/kernel/nfs-root-name " and " +.BR /proc/sys/kernel/nfs-root-addrs +and then writing 0xff (e.g. the pseudo-NFS-device number) into file +.BR /proc/sys/kernel/real-root-dev "." +For example, the following shell command line would change +the normal root device to +.BR /dev/hdb1 : +.nf + echo 0x365 >/proc/sys/kernel/real-root-dev +.fi +For a NFS example, the following shell command lines would change the +normal root device to the NFS directory +.BR /var/nfsroot +on a local networked NFS server with IP number 193.8.232.7 for a system with +IP number 193.8.232.7 and named 'idefix': +.nf + 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 +.fi +.\" +.\" +.\" +.SH USAGE +The main motivation for implementing +.BR initrd +was to allow for modular kernel configuration at system installation. +.PP +A possible system installation scenario is as follows: +.RS 0.2i +.PP +1. The loader program boots from floppy or other media with a minimal kernel +(e.g. support for +.BR /dev/ram ", " /dev/initrd ", and the ext2 file-system) and loads " +.BR /dev/initrd " with a gzipped version of the initial file-system. +.PP +2. The executable +.BR /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. +.PP +3. The executable +.BR /linuxrc +loads the necessary modules from the initial root file-system. +.PP +4. The executable +.BR /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.) +.PP +5. The executable +.BR /linuxrc " sets " /proc/sys/kernel/real-root-dev, +unmount +.BR /proc ", " +the normal root file system and any other file +systems it has mounted, and then terminates. +.PP +6. The kernel then mounts the normal root file system. +.PP +7. Now that the file system is accessible and intact, +the boot loader can be installed. +.PP +8. The boot loader is configured to load into +.BR /dev/initrd +a file system with the set of modules that was used to bring up the system. +(e.g. Device +.BR /dev/ram0 +can be modified, then unmounted, and finally, the image is written from +.BR /dev/ram0 +to a file.) +.PP +9. The system is now bootable and additional installation tasks can be +performed. +.RE +.PP +The key role of +.BR /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. +.PP +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 desirable 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 +.B /linuxrc +file or a file executed by +.B /linuxrc +would be different. +.PP +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 +.B /dev/initrd +can use a dialog and/or auto-detection followed by a +possible sanity check. +.PP +Last but not least, Linux distributions on CD-ROM may use +.BR initrd +for easy installation from the CD-ROM. +The distribution can use +.BR LOADLIN +to directly load +.BR /dev/initrd +from CD-ROM without the need of any floppies. +The distribution could also use a +.BR LILO +boot floppy and then bootstrap a bigger ram disk via +.BR /dev/initrd " from the CD-ROM." +.\" +.\" +.\" +.SH CONFIGURATION +The +.B /dev/initrd +is a read-only block device assigned +major number 1 and minor number 250. +Typically +.B /dev/initrd +is owned by +.B root.disk +with mode 0400 (read access by root only). +If the Linux system does not have +.B /dev/initrd +already created, it can be created with the following commands: +.nf +\fB + mknod -m 400 /dev/initrd b 1 250 + chown root:disk /dev/initrd +\fP +.fi +Also, support for both "RAM disk" and "Initial RAM disk" +(e.g. +.BR CONFIG_BLK_DEV_RAM=y " and " CONFIG_BLK_DEV_INITRD=y +) support must be compiled directly into the Linux kernel to use +.BR /dev/initrd "." +When using +.BR /dev/initrd ", " +the RAM disk driver cannot be loaded as a module. +.\" +.\" +.\" +.SH FILES +.I /dev/initrd +.br +.I /dev/ram0 +.br +.I /linuxrc +.br +.I /initrd +.SH "SEE ALSO" +.BR chown (1), +.BR mknod (1), +.BR ram (4), +.BR freeramdisk (8), +.BR rdev (8) + +The documentation file +.I initrd.txt +in the kernel source package, the LILO documentation, +the LOADLIN documentation, the SYSLINUX documentation. +.\" +.\" +.\" +.SH NOTES +1. With the current kernel, any file systems that remain mounted when +.BR /dev/ram0 " is moved from " / " to " /initrd +continue to be accessible. However, the +.BR /proc/mounts +entries are not updated. +.PP +2. With the current kernel, if directory +.BR /initrd " does not exist, then " /dev/ram0 +will NOT be fully unmounted if +.BR /dev/ram0 +is used by any process or has any file-system mounted on it. +If +.BR /dev/ram0 " is NOT fully unmounted, " +then +.BR /dev/ram0 +will remain in memory. +.PP +3. Users of +.BR /dev/initrd +should not depend on the behavior give in the above notes. +The behavior may change in future versions of the Linux kernel. +.\" +.\" +.\" +.SH AUTHOR +The kernel code for device +.BR initrd +was written by Werner Almesberger <almesber@lrc.epfl.ch> and +Hans Lermen <lermen@elserv.ffm.fgan.de>. +The code for +.BR initrd +was added to the baseline Linux kernel in development version 1.3.73. |