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NAME

       initrd - boot loader initialized RAM disk

DESCRIPTION

       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
       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.

BOOT-UP OPERATION

       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 behavior.)

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

OPTIONS

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

       initrd=filename
              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.

       noinitrd
              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.

       root=device-name
              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 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
       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/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
       /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  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
       networked  NFS  server  with IP number 193.8.232.7 for a system with IP
       number 193.8.232.7 and named ’idefix’:
            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

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       system     is     obsolete.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

USAGE

       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
         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-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 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 /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.

CONFIGURATION

       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.

FILES

       /dev/initrd
       /dev/ram0
       /linuxrc
       /initrd

SEE ALSO

       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
       documentation.

NOTES

       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.

AUTHOR

       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
       development version 1.3.73.