initrd(4) — Linux manual page

NAME | CONFIGURATION | DESCRIPTION | FILES | NOTES | SEE ALSO | COLOPHON

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

NAME         top

       initrd - boot loader initialized RAM disk

CONFIGURATION         top

       /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         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.

   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         top

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

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 5.11 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
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                      INITRD(4)

Pages that refer to this page: pivot_root(2)