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.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 isUNSPECIFIED
. See theNOTES
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 isUNSPECIFIED
. See theNOTES
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=
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.
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:
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
with a gzipped version of the initial file-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 /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.
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.
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.
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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