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

NAME

       initrd - boot loader initialized RAM disk

DESCRIPTION

       The  special  file /dev/initrd is a read-only block device.  Device /dev/initrd is a RAM disk that is initialized (e.g. loaded) by the boot
       loader before the kernel is started.  The kernel then can use the the block device /dev/initrd's contents for a two phased 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 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-
	 systems 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 has directory /initrd, device /dev/ram0 is moved from / to /initrd.  Otherwise if directory /initrd does not
	 exist device /dev/ram0 is unmounted.  (When moved from / to /initrd, /dev/ram0 is not unmounted and therefore processes can  remain  run-
	 ning  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 time 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 or compiled into the kernel file), or the boot loader option set-
       ting is used for the normal root file systems.  For a  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 nfs-
       root.txt.  For more information on setting the root file system also see 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 a 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 a 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

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
	 system)  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 re-use the configuration data during normal system operation without requiring initial ker-
       nel 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  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.

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 ) support 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.

FILES

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

SEE ALSO

       chown(1), mknod(1), /dev/ram(4), freeramdisk(8), rdev(8), The documentation file initrd.txt in the kernel source package, the LILO documen-
       tation, the LOADLIN documentation, the SYSLINUX documentation.

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.

AUTHOR

       The  kernel  code  for  device initrd was written by Werner Almesberger <almesber@lrc.epfl.ch> and Hans Lermen <lermen@elserv.ffm.fgan.de>.
       The code for initrd was added to the baseline Linux kernel in development version 1.3.73.

Linux 2.0							    1997-11-06								 INITRD(4)