initrd - boot loader initialized RAM disk
/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
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) 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.
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 before the
kernel is started. The kernel then can use /dev/initrd's contents 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 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 filesystem (i.e., the
normal root filesystem) is mounted from a different device.
When booting up with initrd, the system boots as follows:
The following boot loader options, when used with initrd, affect the
kernel's boot-up operation:
- The boot loader loads the kernel program and /dev/initrd's contents
- 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.
- The kernel then read-write mounts the device /dev/ram0 as the
initial root filesystem.
- 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.
- 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.)
- 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
- 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 remain running from
/dev/ram0 when /linuxrc exits, the behavior of the kernel is
UNSPECIFIED. See the NOTES section for the current kernel
- The usual boot sequence (e.g., invocation of /sbin/init) is
performed on 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
information on NFS-mounted root see the kernel documentation file
Documentation/filesystems/nfsroot.txt before Linux 2.6.33). For more
information on setting the root filesystem see also the LILO and
- 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.
- 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.
- 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 the
this option must be a mountable device having a suitable root
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
/proc/sys/kernel/nfs-root-addrs. For a physical root device, the root
device is changed by having /linuxrc write the new root filesystem
device number into /proc/sys/kernel/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 command 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
networked NFS server with IP number 22.214.171.124 for a system with IP number
126.96.36.199 and named "idefix":
echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
echo 188.8.131.52:184.108.40.206::255.255.255.0:idefix \
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
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.
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
filesystem) and loads /dev/initrd with a gzipped version of the
- The executable /linuxrc determines what is needed to (1) mount the
normal root filesystem (i.e., device type, device drivers, filesystem) 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
- The executable /linuxrc loads the necessary modules from the
initial root filesystem.
- The executable /linuxrc creates and populates the root filesystem.
(At this stage the normal root filesystem does not have to be a completed
- The executable /linuxrc sets /proc/sys/kernel/real-root-dev,
unmount /proc, the normal root filesystem and any other filesystems
it has mounted, and then terminates.
- The kernel then mounts the normal root filesystem.
- Now that the filesystem is accessible and intact, the boot loader can be
- 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 finally, the
image is written from /dev/ram0 to a file.)
- The system is now bootable and additional installation tasks can be
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 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 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
chown(1), mknod(1), ram(4), freeramdisk(8),
- 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.
- 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.
- 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
Documentation/initrd.txt before Linux 4.10) in the Linux kernel
source tree, the LILO documentation, the LOADLIN documentation, the SYSLINUX
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