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

           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.


   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

   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
          the 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/nfsroot.txt.  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
   Documentation/initrd.txt 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 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, unmount
      /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

   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.




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


   chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)

   Documentation/initrd.txt in the Linux  kernel  source  tree,  the  LILO
   documentation, the LOADLIN documentation, the SYSLINUX documentation


   This  page  is  part of release 4.09 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


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