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.
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) initrd is mounted read-write as root
4) /linuxrc 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)
5) linuxrc mounts the "real" root file system
6) linuxrc places the root file system at the root directory
using the pivot_root system call
7) the usual boot sequence (e.g. invocation of /sbin/init) is
performed on the root file system
8) 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.
Usage scenarios
---------------
The main motivation for implementing initrd was to allow
for modular kernel configuration at system installation.
The procedure would work as follows:
1) system boots from floppy or other media with a minimal kernel
(e.g. support for RAM disks, initrd, a.out, and the Ext2 FS)
and loads initrd
2) /linuxrc determines what is needed to (1) mount the "real" root
FS (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) /linuxrc loads the necessary kernel modules
4) /linuxrc creates and populates the root file system (this
doesn't have to be a very usable system yet)
5) /linuxrc invokes pivot_root to change the root file system and
execs - via chroot - a program that continues the installation
6) the boot loader is installed
7) the boot loader is configured to load an initrd with the set of
modules that was used to bring up the system (e.g. /initrd can
be modified, then unmounted, and finally, the image is written
from /dev/ram0 or /dev/rd/0 to a file)
8) now the system is bootable and additional installation tasks
can be performed
The key role of initrd here is to re-use the configuration data
during normal system operation without requiring the use of a
bloated "generic" kernel or re-compiling or re-linking the kernel.
A second scenario is for installations where Linux runs on systems
with different hardware configurations in a single administrative
domain. In such cases, it is desirable to generate 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, a
common initrd could be generated with all the necessary modules.
Then, only /linuxrc or a file read by it would have to be different.
A third scenario are more convenient recovery disks, because
information like the location of the root FS partition doesn't have
to be provided at boot time, but the system loaded from initrd can
invoke a user-friendly dialog and it can also perform some sanity
checks (or even some form of auto-detection).
Last not least, CD-ROM distributors may use it for better installation
from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
via initrd from CD; or by booting via a loader like LOADLIN or directly
from the CD-ROM, and loading the RAM disk from CD without need of floppies.
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