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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 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,  unmounts /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

       •  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.

       •  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.

       •  Users  of  /dev/initrd should not depend on the behavior given 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