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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 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 ini-
       tialized 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 con-
	 tents 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  nor-
	 mal  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 behavior.)

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

       The following boot loader options when used  with  initrd,  affect  the
       kernel's 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.

       By  default,  the  kernel's  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 com-
       mand 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 sys-
	 tem) 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  ini-
	 tial root file-system.

	 4.  The  executable /linuxrc creates and populates the root file sys-
	 tem.  (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 sys-
	 tem  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 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.

       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.
       CONFIG_BLK_DEV_RAM=y and CONFIG_BLK_DEV_INITRD=y ) support must be com-
       piled  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), 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-

       1. With the current kernel, any file systems that remain	 mounted  when
       /dev/ram0  is  moved from / to /initrd continue to be accessible.  How-
       ever, 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 pro-
       cess 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

       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 baseline Linux kernel in develop-
       ment version 1.3.73.

Linux 2.0			  1997-11-06			     INITRD(4)
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