INITRD (4) | Special files and drivers | Unix Manual Pages

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 contents. 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 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"youhavetousethiscommandinthe 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"thisisaboottimeoptionor 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:

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"withagzippedversionoftheinitialfile-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 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."

NOTES

AUTHOR

Created by Blin Media, 2008-2013

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