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INITRD(4)                  Linux Programmer's 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/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.

   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 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
       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.

   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  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
          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)