Operation¶

When using initrd, the system typically boots as follows:

1. the boot loader loads the kernel and the initial RAM disk
2. the kernel converts initrd into a “normal” RAM disk andfrees the memory used by initrd
3. if the root device is not/dev/ram0, the old (deprecated)change_root procedure is followed. see the “Obsolete root changemechanism” section below.
4. root device is mounted. if it is/dev/ram0, the initrd image isthen mounted as root
5. /sbin/init is executed (this can be any valid executable, includingshell scripts; it is run with uid 0 and can do basically everythinginit can do).
6. init mounts the “real” root file system
7. init places the root file system at the root directory using thepivot_root system call
8. init execs the/sbin/init on the new root filesystem, performingthe usual boot sequence
9. the initrd file system is removed

Note that changing the root directory does not involve unmounting it.It is therefore possible to leave processes running on initrd during thatprocedure. Also note that file systems mounted under initrd continue tobe accessible.

Boot command-line options¶

initrd adds the following new options:

initrd=<path>    (e.g. LOADLIN)  Loads the specified file as the initial RAM disk. When using LILO, you  have to specify the RAM disk image file in /etc/lilo.conf, using the  INITRD configuration variable.noinitrd  initrd data is preserved but it is not converted to a RAM disk and  the "normal" root file system is mounted. initrd data can be read  from /dev/initrd. Note that the data in initrd can have any structure  in this case and doesn't necessarily have to be a file system image.  This option is used mainly for debugging.  Note: /dev/initrd is read-only and it can only be used once. As soon  as the last process has closed it, all data is freed and /dev/initrd  can't be opened anymore.root=/dev/ram0  initrd is mounted as root, and the normal boot procedure is followed,  with the RAM disk mounted as root.

Compressed cpio images¶

Recent kernels have support for populating a ramdisk from a compressed cpioarchive. On such systems, the creation of a ramdisk image doesn’t need toinvolve special block devices or loopbacks; you merely create a directory ondisk with the desired initrd content, cd to that directory, and run (as anexample):

find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img

Examining the contents of an existing image file is just as simple:

mkdir /tmp/imagefilecd /tmp/imagefilegzip -cd /boot/imagefile.img | cpio -imd --quiet

Installation¶

First, a directory for the initrd file system has to be created on the“normal” root file system, e.g.:

# mkdir /initrd

The name is not relevant. More details can be found on thepivot_root(2) man page.

If the root file system is created during the boot procedure (i.e. ifyou’re building an install floppy), the root file system creationprocedure should create the/initrd directory.

If initrd will not be mounted in some cases, its content is stillaccessible if the following device has been created:

# mknod /dev/initrd b 1 250# chmod 400 /dev/initrd

Second, the kernel has to be compiled with RAM disk support and withsupport for the initial RAM disk enabled. Also, at least all componentsneeded to execute programs from initrd (e.g. executable format and filesystem) must be compiled into the kernel.

Third, you have to create the RAM disk image. This is done by creating afile system on a block device, copying files to it as needed, and thencopying the content of the block device to the initrd file. With recentkernels, at least three types of devices are suitable for that:

• a floppy disk (works everywhere but it’s painfully slow)
• a RAM disk (fast, but allocates physical memory)
• a loopback device (the most elegant solution)

We’ll describe the loopback device method:

1. make sure loopback block devices are configured into the kernel

2. create an empty file system of the appropriate size, e.g.:

   # dd if=/dev/zero of=initrd bs=300k count=1# mke2fs -F -m0 initrd

   (if space is critical, you may want to use the Minix FS instead of Ext2)

3. mount the file system, e.g.:

   # mount -t ext2 -o loop initrd /mnt

4. create the console device:

   # mkdir /mnt/dev# mknod /mnt/dev/console c 5 1

5. copy all the files that are needed to properly use the initrdenvironment. Don’t forget the most important file,/sbin/init

   Note

   /sbin/init permissions must include “x” (execute).

6. correct operation the initrd environment can frequently be testedeven without rebooting with the command:

   # chroot /mnt /sbin/init

   This is of course limited to initrds that do not interfere with thegeneral system state (e.g. by reconfiguring network interfaces,overwriting mounted devices, trying to start already running demons,etc. Note however that it is usually possible to use pivot_root insuch a chroot’ed initrd environment.)

7. unmount the file system:

   # umount /mnt

8. the initrd is now in the file “initrd”. Optionally, it can now becompressed:

   # gzip -9 initrd

For experimenting with initrd, you may want to take a rescue floppy andonly add a symbolic link from/sbin/init to/bin/sh. Alternatively, youcan try the experimental newlib environment[2] to create a smallinitrd.

Finally, you have to boot the kernel and load initrd. Almost all Linuxboot loaders support initrd. Since the boot process is still compatiblewith an older mechanism, the following boot command line parametershave to be given:

root=/dev/ram0 rw

(rw is only necessary if writing to the initrd file system.)

With LOADLIN, you simply execute:

LOADLIN <kernel> initrd=<disk_image>

e.g.:

LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw

With LILO, you add the optionINITRD=<path> to either the global sectionor to the section of the respective kernel in/etc/lilo.conf, and passthe options using APPEND, e.g.:

image = /bzImage  initrd = /boot/initrd.gz  append = "root=/dev/ram0 rw"

and run/sbin/lilo

For other boot loaders, please refer to the respective documentation.

Now you can boot and enjoy using initrd.

Changing the root device¶

When finished with its duties, init typically changes the root deviceand proceeds with starting the Linux system on the “real” root device.

The procedure involves the following steps:

• mounting the new root file system
• turning it into the root file system
• removing all accesses to the old (initrd) root file system
• unmounting the initrd file system and de-allocating the RAM disk

Mounting the new root file system is easy: it just needs to be mounted ona directory under the current root. Example:

# mkdir /new-root# mount -o ro /dev/hda1 /new-root

The root change is accomplished with the pivot_root system call, whichis also available via thepivot_root utility (seepivot_root(8)man page;pivot_root is distributed with util-linux version 2.10h or higher[3]).pivot_root moves the current root to a directory under the newroot, and puts the new root at its place. The directory for the old rootmust exist before callingpivot_root. Example:

# cd /new-root# mkdir initrd# pivot_root . initrd

Now, the init process may still access the old root via itsexecutable, shared libraries, standard input/output/error, and itscurrent root directory. All these references are dropped by thefollowing command:

# exec chroot . what-follows <dev/console >dev/console 2>&1

Where what-follows is a program under the new root, e.g./sbin/initIf the new root file system will be used with udev and has no valid/dev directory, udev must be initialized before invoking chroot in orderto provide/dev/console.

Note: implementation details of pivot_root may change with time. In orderto ensure compatibility, the following points should be observed:

• before calling pivot_root, the current directory of the invokingprocess should point to the new root directory
• use . as the first argument, and the _relative_ path of the directoryfor the old root as the second argument
• a chroot program must be available under the old and the new root
• chroot to the new root afterwards
• use relative paths for dev/console in the exec command

Now, the initrd can be unmounted and the memory allocated by the RAMdisk can be freed:

# umount /initrd# blockdev --flushbufs /dev/ram0

It is also possible to use initrd with an NFS-mounted root, see thepivot_root(8) man page for details.

Usage scenarios¶

The main motivation for implementing initrd was to allow for modularkernel configuration at system installation. The procedure would workas follows:

1. system boots from floppy or other media with a minimal kernel(e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) andloads initrd
2. /sbin/init determines what is needed to (1) mount the “real” root FS(i.e. device type, device drivers, file system) and (2) thedistribution media (e.g. CD-ROM, network, tape, …). This can bedone by asking the user, by auto-probing, or by using a hybridapproach.
3. /sbin/init loads the necessary kernel modules
4. /sbin/init creates and populates the root file system (this doesn’thave to be a very usable system yet)
5. /sbin/init invokespivot_root to change the root file system andexecs - via chroot - a program that continues the installation
6. the boot loader is installed
7. the boot loader is configured to load an initrd with the set ofmodules that was used to bring up the system (e.g./initrd can bemodified, then unmounted, and finally, the image is written from/dev/ram0 or/dev/rd/0 to a file)
8. now the system is bootable and additional installation tasks can beperformed

The key role of initrd here is to re-use the configuration data duringnormal system operation without requiring the use of a bloated “generic”kernel or re-compiling or re-linking the kernel.

A second scenario is for installations where Linux runs on systems withdifferent hardware configurations in a single administrative domain. Insuch cases, it is desirable to generate only a small set of kernels(ideally only one) and to keep the system-specific part of configurationinformation as small as possible. In this case, a common initrd could begenerated with all the necessary modules. Then, only/sbin/init or a fileread by it would have to be different.

A third scenario is more convenient recovery disks, because informationlike the location of the root FS partition doesn’t have to be provided atboot time, but the system loaded from initrd can invoke a user-friendlydialog and it can also perform some sanity checks (or even some form ofauto-detection).

Last not least, CD-ROM distributors may use it for better installationfrom CD, e.g. by using a boot floppy and bootstrapping a bigger RAM diskvia initrd from CD; or by booting via a loader likeLOADLIN or directlyfrom the CD-ROM, and loading the RAM disk from CD without need offloppies.

Obsolete root change mechanism¶

The following mechanism was used before the introduction of pivot_root.Current kernels still support it, but you should _not_ rely on itscontinued availability.

It works by mounting the “real” root device (i.e. the one set with rdevin the kernel image or with root=… at the boot command line) as theroot file system when linuxrc exits. The initrd file system is thenunmounted, or, if it is still busy, moved to a directory/initrd, ifsuch a directory exists on the new root file system.

In order to use this mechanism, you do not have to specify the bootcommand options root, init, or rw. (If specified, they will affectthe real root file system, not the initrd environment.)

If /proc is mounted, the “real” root device can be changed from withinlinuxrc by writing the number of the new root FS device to the specialfile /proc/sys/kernel/real-root-dev, e.g.:

# echo 0x301 >/proc/sys/kernel/real-root-dev

Note that the mechanism is incompatible with NFS and similar filesystems.

This old, deprecated mechanism is commonly calledchange_root, whilethe new, supported mechanism is calledpivot_root.

Mixed change_root and pivot_root mechanism¶

In case you did not want to useroot=/dev/ram0 to trigger the pivot_rootmechanism, you may create both/linuxrc and/sbin/init in your initrdimage.

/linuxrc would contain only the following:

#! /bin/shmount -n -t proc proc /procecho 0x0100 >/proc/sys/kernel/real-root-devumount -n /proc

Once linuxrc exited, the kernel would mount again your initrd as root,this time executing/sbin/init. Again, it would be the duty of this initto build the right environment (maybe using theroot=device passed onthe cmdline) before the final execution of the real/sbin/init.

Resources¶