initrd(4) — Linux manual page


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

NAME         top

       initrd - boot loader initialized RAM disk


       /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., 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.

DESCRIPTION         top

       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.

       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 filesystem 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 filesystem 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 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

   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 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
       filesystem is obsolete.  See the Linux kernel source file Documenta‐
       tion/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.

       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 ini‐
          tial 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 filesys‐
          tems 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 filesys‐
          tem 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 configura‐
       tion 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 informa‐
       tion 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         top


NOTES         top

       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 given in
          the above notes.  The behavior may change in future versions of
          the Linux kernel.

SEE ALSO         top

       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         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-03-06                        INITRD(4)

Pages that refer to this page: pivot_root(2)