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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.  This  device
       is  a  RAM  disk  that is initialized (e.g., loaded) by the boot loader
       before the kernel is started.  The kernel then  can  use  /dev/initrd's
       contents for a two-phase 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 the device /dev/ram0 as the ini-
          tial 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 system has a directory /initrd, the device
          /dev/ram0 is moved from / to /initrd.  Otherwise  if  the  directory
          /initrd  does  not  exist, the 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 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 an 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 Documenta-
       tion/filesystems/nfsroot.txt.  For more information on setting the root
       file system see also 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  an  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 an 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 reuse 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 3.25 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                             2009-04-04                         INITRD(4)

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