initrd — boot loader initialized RAM disk
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 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.
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. IP 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.
The following boot loader options when used with
initrd, affect
the kernel's boot-up operation:
initrd=filenameSpecifies 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.
noinitrdThis 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-nameSpecifies 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.
By default, the kernel's settings (e.g. set in the
kernel file with rdev(8) 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
Note: The use
of /proc/sys/kernel/real-root-dev to change
the root file system is obsolete. See the kernel source
file Documentation/initrd.txt as
well as pivot_root(2) and
pivot_root(8) for
information on the modern method of changing the root file
system.
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.
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
/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.
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.
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.
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.
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.
chown(1), mknod(1), 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.
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