INITRD(4)                                           Linux Programmer's Manual                                          INITRD(4)



NAME
       initrd - boot loader initialized RAM disk

CONFIGURATION
       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 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 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 ker-
          nel 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.

   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 file-system 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 file system image.  However, device /dev/initrd is read-only and can be read only one time after sys-
              tem startup.

       root=device-name
              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  physi-
       cal  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/nfs-
       root on a local networked NFS server with IP number 193.8.232.7 for a system with IP number 193.8.232.7 and  named  "ide-
       fix":

           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 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.

   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 file-system) and loads /dev/initrd with a gzipped version of the initial file-system.

       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 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 file-system parti-
       tion  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 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.

SEE ALSO
       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 documentation.

COLOPHON
       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)