Originální popis anglicky:
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 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 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=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
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
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 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.
/dev/initrd
/dev/ram0
/linuxrc
/initrd
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.
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.
