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       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.
       CONFIG_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
       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  the  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 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
          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 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
       information  on  NFS-mounted  root  see  the  kernel documentation file
       Documentation/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
       /linuxrc   write   the   new   root  file  system  device  number  into
       /proc/sys/kernel/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
       command 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
       networked  NFS  server  with IP number for a system with IP
       number 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/initrd.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
       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.




       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


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