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NAME

       initrd - boot loader initialized RAM disk

CONFIGURATION

       /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)   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.

DESCRIPTION

       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  filesystem
       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 filesystem (i.e., the
       normal root filesystem) 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 filesystem.

       4. If  the  indicated  normal  root  filesystem is also the initial root filesystem (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 filesystem, /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 filesystem  is
          mounted.   (If  /linuxrc  exits  with  any  filesystems  mounted  on  the  initial root
          filesystem, then the behavior of the kernel is UNSPECIFIED.  See the NOTES section  for
          the current kernel behavior.)

       7. If  the  normal  root filesystem 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 filesystem 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 filesystem.

   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 filesystem image.

       noinitrd
              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 filesystem 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 filesystem.  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 filesystem.

   Changing the normal root filesystem
       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
       filesystems.  For an NFS-mounted normal root filesystem, 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/nfs/nfsroot.txt
       (or  Documentation/filesystems/nfsroot.txt  before Linux 2.6.33).  For more information on
       setting the root filesystem 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 filesystem device number into /proc/sys/kernel/real-root-dev.  For an
       NFS root filesystem, 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
       193.8.232.7 for a system with IP number 193.8.232.2 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 filesystem is obsolete.
       See    the    Linux    kernel   source   file   Documentation/admin-guide/initrd.rst   (or
       Documentation/initrd.txt before Linux 4.10) as well as pivot_root(2) and pivot_root(8) for
       information on the modern method of changing the root filesystem.

   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 filesystem) and loads /dev/initrd with
          a gzipped version of the initial filesystem.

       2. The executable /linuxrc determines  what  is  needed  to  (1)  mount  the  normal  root
          filesystem  (i.e.,  device  type,  device drivers, filesystem) 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 filesystem.

       4. The  executable /linuxrc creates and populates the root filesystem.  (At this stage the
          normal root filesystem 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 filesystem and any other filesystems it has mounted, and then terminates.

       6. The kernel then mounts the normal root filesystem.

       7. Now that the filesystem is accessible and intact, the boot loader can be installed.

       8. The  boot  loader  is  configured to load into /dev/initrd a filesystem 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 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  filesystem  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.

FILES

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

NOTES

       1. With  the  current  kernel, any filesystems 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 filesystem 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.

SEE ALSO

       chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)

       Documentation/admin-guide/initrd.rst (or Documentation/initrd.txt before  Linux  4.10)  in
       the  Linux  kernel  source  tree,  the  LILO documentation, the LOADLIN documentation, the
       SYSLINUX documentation

COLOPHON

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