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  con-
       tents.	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-sys-
	  tems 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 pro-
	  cesses  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 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 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/ini-
	      trd 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 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  Documen-
       tation/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 physical 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/nfsroot on a local net-
       worked NFS server with IP number 193.8.232.7 for a system with IP number 193.8.232.7 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 file system is obsolete.  See the kernel source file	Documentation/ini-
       trd.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 sys-
	  tem)	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  net-
       work.   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 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

       /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.  How-
	  ever, 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 documenta-
       tion.

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)