# initrd

## NAME



           initrd - boot loader initialized RAM disk
    


## CONFIGURATION



           _/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**) 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.
    


## 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 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.
    
       **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
                initial 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 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 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 information on NFS-mounted root see the kernel
           documentation file _Documentation/filesystems/nfs/nfsroot.txt_ (or
           _Documentation/filesystems/nfsroot.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 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
           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 193.8.232.7 for a system
           with IP number 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
           _Documentation/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 initial filesystem.
    
           (2)  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 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 completed system yet.)
    
           (5)  The executable _/linuxrc_ sets _/proc/sys/kernel/real-root-dev_ ,
                unmounts _/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 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 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 CD-ROM.
    


## FILES



           _/dev/initrd_
           _/dev/ram0_
           _/linuxrc_
           _/initrd_
    


## NOTES



           •  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 given in
              the above notes.  The behavior may change in future versions of
              the Linux kernel.
    


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