litmus-rt.git - The LITMUS^RT kernel.

Using the initial RAM disk (initrd)
===================================

Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
                     Hans Lermen <lermen@fgan.de>


initrd provides the capability to load a RAM disk by the boot loader.
This RAM disk can then be mounted as the root file system and programs
can be run from it. Afterwards, a new root file system can be mounted
from a different device. The previous root (from initrd) is then moved
to a directory and can be subsequently unmounted.

initrd is mainly designed to allow system startup to occur in two phases,
where the kernel comes up with a minimum set of compiled-in drivers, and
where additional modules are loaded from initrd.

This document gives a brief overview of the use of initrd. A more detailed
discussion of the boot process can be found in [1].


Operation
---------

When using initrd, the system typically boots as follows:

  1) the boot loader loads the kernel and the initial RAM disk
  2) the kernel converts initrd into a "normal" RAM disk and
     frees the memory used by initrd
  3) if the root device is not /dev/ram0, the old (deprecated)
     change_root procedure is followed. see the "Obsolete root change
     mechanism" section below.
  4) root device is mounted. if it is /dev/ram0, the initrd image is
     then mounted as root
  5) /sbin/init is executed (this can be any valid executable, including
     shell scripts; it is run with uid 0 and can do basically everything
     init can do).
  6) init mounts the "real" root file system
  7) init places the root file system at the root directory using the
     pivot_root system call
  8) init execs the /sbin/init on the new root filesystem, performing
     the usual boot sequence
  9) the initrd file system is removed

Note that changing the root directory does not involve unmounting it.
It is therefore possible to leave processes running on initrd during that
procedure. Also note that file systems mounted under initrd continue to
be accessible.


Boot command-line options
-------------------------

initrd adds the following new options:

  initrd=<path>    (e.g. LOADLIN)

    Loads the specified file as the initial RAM disk. When using LILO, you
    have to specify the RAM disk image file in /etc/lilo.conf, using the
    INITRD configuration variable.

  noinitrd


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