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/initrdonto device/dev/ram0and then frees the memory used by/dev/initrd.3. The kernel then read-write mounts device
/dev/ram0as 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
/linuxrcis present in the initial root file-system,/linuxrcis executed with UID 0. (The file/linuxrcmust have executable permission. The file/linuxrccan be any valid executable, including a shell script.)6. If
/linuxrcis not executed or when/linuxrcterminates, the normal root file system is mounted. (If/linuxrcexits with any file-systems mounted on the initial root file-system, then the behavior of the kernel isUNSPECIFIED. See theNOTESsection for the current kernel behavior.)7. If the normal root file has directory
/initrd, device/dev/ram0is moved from/to/initrd. Otherwise if directory/initrddoes not exist device/dev/ram0is unmounted. (When moved from/to/initrd,/dev/ram0is not unmounted and therefore processes can remain running from/dev/ram0. If directory/initrddoes not exist on the normal root file-system and any processes remain running from/dev/ram0when/linuxrcexits, the behavior of the kernel isUNSPECIFIED. See theNOTESsection 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:
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/initrdwith a gzipped version of the initial file-system.2. The executable
/linuxrcdetermines 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
/linuxrcloads the necessary modules from the initial root file-system.4. The executable
/linuxrccreates 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
/linuxrcsets/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/initrda file system with the set of modules that was used to bring up the system. (e.g. Device/dev/ram0can be modified, then unmounted, and finally, the image is written from/dev/ram0to 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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