Quick ?s
Cheat Sheets
Man Pages
The Lynx
INITRD(4)		   Linux Programmers Manual		    INITRD(4)

       initrd - boot loader initialized RAM disk

       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

	       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.  CON
       FIG_BLK_DEV_RAM=y  and  CONFIG_BLK_DEV_INITRD=y)   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.


       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
       block device /dev/initrds 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  ini
       tialized  by the boot loader).  In the second phase, additional drivers
       or other modules are loaded from the initial  root  devices  contents.
       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/initrds  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 exe
	  cutable, 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-sys
	  tems	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  run
	  ning	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 UNSPEC
	  IFIED.  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.

       The  following  boot  loader  options when used with initrd, affect the
       kernels boot-up operation:

	      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.

	      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/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.

   Changing the Normal Root File System
       By default, the kernels settings (e.g. set in  the  kernel  file  with
       rdev(8)	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	infor
       mation  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  /lin
       uxrc  write  the new root file system device number into /proc/sys/ker
       nel/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  net
       worked  NFS server with IP number for a system with IP num
       ber and named "idefix":

	   echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
	   echo \
	   echo 255 >/proc/sys/kernel/real-root-dev

       Note: The use of /proc/sys/kernel/real-root-dev to change the root file
       system  is  obsolete.   See  the  kernel source file Documentation/ini
       trd.txt as well as pivot_root(2) and pivot_root(8) for  information  on
       the modern method of changing the root file system.

       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

       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 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

       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  configura
       tion  information  as small as possible.  In this case, create a common
       file with all needed modules.  Then, only the /linuxrc file or  a  file
       executed by /linuxrc would be different.

       A  third  scenario is more convenient recovery disks.  Because informa
       tion 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.

       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.

       chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)

       The  documentation  file  initrd.txt  in the kernel source package, the
       LILO documentation, the LOADLIN documentation, the SYSLINUX  documenta

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2007-11-26			     INITRD(4)

Yals.net is © 1999-2009 Crescendo Communications
Sharing tech info on the web for more than a decade!
This page was generated Thu Apr 30 17:05:30 2009