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!!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 FileSystem from the contents  of  __/dev/initrd __ (e.g. RAM disk initialized by the BootLoader).  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 file system  (i.e. the normal root file system) is [Mount]ed from a  different device.

!!BOOT-UP OPERATION
When booting up with  __initrd__ , the system boots as follows:

# The boot loader loads the kernel program and   __/dev/initrd__ 's contents into memory.
# 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__ .
# The kernel then read-write mounts device  __/dev/ram0 __  as the initial root file system.
# 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.
# 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.)
# 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.)
# 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 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.)
# 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/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 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 setting 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 __nfsroot.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 networked 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:

# 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.
# 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.
# The executable   __/linuxrc__  loads the necessary modules from the initial root file-system.
# 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.)
# 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.
# The kernel then mounts the normal root file system.
# Now that the file system is accessible and intact,  the boot loader can be installed.
# 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.)
# 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 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 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 documentation, 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.  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 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.
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Last edited on Monday, December 29, 2003 11:35:43 am by CraigBox
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