INITRD(4) Linux Programmer's Manual INITRD(4)
NAME
initrd - boot loader initialized RAM disk
CONFIGURATION
/dev/initrd is a read-only block device assigned major number 1 and mi-
nor 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 com-
mands:
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., CON-
FIG_BLK_DEV_RAM=y and CONFIG_BLK_DEV_INITRD=y) must be compiled di-
rectly into the Linux kernel to use /dev/initrd. When using /dev/ini-
trd, the RAM disk driver cannot be loaded as a module.
DESCRIPTION
The special file /dev/initrd is a read-only block device. This device
is a RAM disk that is initialized (e.g., loaded) by the boot loader be-
fore the kernel is started. The kernel then can use /dev/initrd's con-
tents for a two-phase system boot-up.
In the first boot-up phase, the kernel starts up and mounts an initial
root filesystem from the contents of /dev/initrd (e.g., RAM disk ini-
tialized 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 filesystem (i.e., the
normal root filesystem) is mounted from a different device.
Boot-up operation
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 the device /dev/ram0 as the ini-
tial root filesystem.
4. If the indicated normal root filesystem is also the initial root
filesystem (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
filesystem, /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 filesystem is mounted. (If /linuxrc exits with any filesystems
mounted on the initial root filesystem, then the behavior of the
kernel is UNSPECIFIED. See the NOTES section for the current kernel
behavior.)
7. If the normal root filesystem has a directory /initrd, the device
/dev/ram0 is moved from / to /initrd. Otherwise, if the directory
/initrd does not exist, the 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 filesystem and any processes re-
main 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 per-
formed on the normal root filesystem.
Options
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 filesystem image.
noinitrd
This boot 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 filesystem 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 filesystem.
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
this option must be a mountable device having a suitable root
filesystem.
Changing the normal root filesystem
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 filesystems. For an NFS-mounted
normal root filesystem, one has to use the nfs_root_name and
nfs_root_addrs boot options to give the NFS settings. For more infor-
mation on NFS-mounted root see the kernel documentation file Documenta-
tion/filesystems/nfs/nfsroot.txt (or Documentation/filesystems/nfs-
root.txt before Linux 2.6.33). For more information on setting the
root filesystem see also 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 de-
vice 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 /lin-
uxrc write the new root filesystem device number into /proc/sys/ker-
nel/real-root-dev. For an NFS root filesystem, 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 com-
mand line would change the normal root device to /dev/hdb1:
echo 0x365 >/proc/sys/kernel/real-root-dev
For an NFS example, the following shell command lines would change the
normal root device to the NFS directory /var/nfsroot on a local net-
worked NFS server with IP number 193.8.232.7 for a system with IP num-
ber 193.8.232.2 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
filesystem is obsolete. See the Linux kernel source file Documenta-
tion/admin-guide/initrd.rst (or Documentation/initrd.txt before Linux
4.10) as well as pivot_root(2) and pivot_root(8) for information on the
modern method of changing the root filesystem.
Usage
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
filesystem) and loads /dev/initrd with a gzipped version of the ini-
tial filesystem.
2. The executable /linuxrc determines what is needed to (1) mount the
normal root filesystem (i.e., device type, device drivers, filesys-
tem) 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 filesystem.
4. The executable /linuxrc creates and populates the root filesystem.
(At this stage the normal root filesystem does not have to be a com-
pleted system yet.)
5. The executable /linuxrc sets /proc/sys/kernel/real-root-dev, un-
mounts /proc, the normal root filesystem and any other filesystems
it has mounted, and then terminates.
6. The kernel then mounts the normal root filesystem.
7. Now that the filesystem is accessible and intact, the boot loader
can be installed.
8. The boot loader is configured to load into /dev/initrd a filesystem
with the set of modules that was used to bring up the system.
(e.g., device /dev/ram0 can be modified, then unmounted, and fi-
nally, 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 reuse 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 configura-
tion 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 informa-
tion like the location of the root filesystem 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.
FILES
/dev/initrd
/dev/ram0
/linuxrc
/initrd
NOTES
1. With the current kernel, any filesystems 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 filesystem 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 given in the
above notes. The behavior may change in future versions of the
Linux kernel.
SEE ALSO
chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)
Documentation/admin-guide/initrd.rst (or Documentation/initrd.txt be-
fore Linux 4.10) in the Linux kernel source tree, the LILO documenta-
tion, the LOADLIN documentation, the SYSLINUX documentation
COLOPHON
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description of the project, information about reporting bugs, and the
latest version of this page, can be found at
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Linux 2019-03-06 INITRD(4)