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RedHat 9 (Linux i386) - man page for initrd (redhat section 4)

INITRD(4)			    Linux Programmer's Manual				INITRD(4)

       initrd - boot loader initialized RAM disk

       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 sys-
       tem 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 contents.  After loading the additional modules, a new root file system (i.e. the
       normal root file system) is mounted from a different device.

       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 terminates, the normal root file  system
	 is  mounted.  (If /linuxrc exits with any file-systems mounted on the initial 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

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

       The following boot loader options when used with initrd, affect the kernel's boot-up oper-

	      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  configura-
	      tion file /etc/lilo.config.  The filename specified with this option will typically
	      be a gzipped file-system image.

	      This boot time option disables the two phase boot-up operation.	The  kernel  per-
	      forms  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/initrd	is  read-
	      only and can be read only one time after system startup.

	      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.

       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 sys-
       tems.  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 nfsroot.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/ker-
       nel/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 system 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 exam-
       ple, 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 number for a system with IP number and named 'idefix':
	    echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
	    echo \
	    echo 255 >/proc/sys/kernel/real-root-dev

       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. sup-
	 port 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-sys-

	 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  nor-
	 mal root file system and any other file systems it has mounted, and then terminates.

	 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/initrd 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 installation 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 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 /linuxrc file or a file executed by /linuxrc would
       be different.

       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 distribution 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.

       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 commands:
	       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.


       chown(1), mknod(1), /dev/ram(4), freeramdisk(8),  rdev(8),  The	documentation  file  ini-
       trd.txt	in  the kernel source package, the LILO documentation, the LOADLIN documentation,
       the SYSLINUX documentation.

       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

       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.

       The kernel code for device initrd was written by Werner Almesberger <almesber@lrc.epfl.ch>
       and Hans Lermen <lermen@elserv.ffm.fgan.de>.  The code for initrd was added to  the  base-
       line Linux kernel in development version 1.3.73.

Linux 2.0				    1997-11-06					INITRD(4)

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