phpman > man > initrd(4)

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



NAME
       initrd - boot loader initialized RAM disk

CONFIGURATION
       /dev/initrd  is a read-only block device assigned major number 1 and minor number 250.  Typi‐
       cally /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 CON‐‐
       FIG_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  con‐
       tents.   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/ini‐
          trd 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 ex‐
          ecuted 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 pro‐
          cesses can remain running from /dev/ram0.  If directory /initrd does not exist on the nor‐
          mal  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  opera‐
       tion:

       initrd=filename
              Specifies the file to load as the contents of /dev/initrd.  For LOADLIN this is a com‐
              mand-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 con‐
              tents 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_ad‐‐
       drs  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    Documenta‐
       tion/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  /lin‐
       uxrc 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 de‐
       vice, the root device is changed by having /linuxrc write the new root filesystem device num‐
       ber  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 num‐
       ber) 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/ini‐
       trd.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 us‐
          ing 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  mod‐
          ules  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, re‐
       compiling 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  con‐
       figuration  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 differ‐
       ent.

       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  with‐
       out  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 up‐
          dated.

       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  be‐
          havior 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
       This  page  is  part  of  release  5.10 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                                        2019-03-06                                    INITRD(4)