Documentation/filesystems/ramfs-rootfs-initramfs.txt - arm/linux - Git at Google


            
            ramfs, rootfs and initramfs

            
            October 17, 2005

            
            Rob Landley <rob@landley.net>

            
            =============================


            What is ramfs?

            
            --------------


            Ramfs is a very simple filesystem that exports Linux's disk caching

            
            mechanisms (the page cache and dentry cache) as a dynamically resizable

            
            RAM-based filesystem.


            Normally all files are cached in memory by Linux.  Pages of data read from

            
            backing store (usually the block device the filesystem is mounted on) are kept

            
            around in case it's needed again, but marked as clean (freeable) in case the

            
            Virtual Memory system needs the memory for something else.  Similarly, data

            
            written to files is marked clean as soon as it has been written to backing

            
            store, but kept around for caching purposes until the VM reallocates the

            
            memory.  A similar mechanism (the dentry cache) greatly speeds up access to

            
            directories.


            With ramfs, there is no backing store.  Files written into ramfs allocate

            
            dentries and page cache as usual, but there's nowhere to write them to.

            
            This means the pages are never marked clean, so they can't be freed by the

            
            VM when it's looking to recycle memory.


            The amount of code required to implement ramfs is tiny, because all the

            
            work is done by the existing Linux caching infrastructure.  Basically,

            
            you're mounting the disk cache as a filesystem.  Because of this, ramfs is not

            
            an optional component removable via menuconfig, since there would be negligible

            
            space savings.


            ramfs and ramdisk:

            
            ------------------


            The older "ram disk" mechanism created a synthetic block device out of

            
            an area of RAM and used it as backing store for a filesystem.  This block

            
            device was of fixed size, so the filesystem mounted on it was of fixed

            
            size.  Using a ram disk also required unnecessarily copying memory from the

            
            fake block device into the page cache (and copying changes back out), as well

            
            as creating and destroying dentries.  Plus it needed a filesystem driver

            
            (such as ext2) to format and interpret this data.


            Compared to ramfs, this wastes memory (and memory bus bandwidth), creates

            
            unnecessary work for the CPU, and pollutes the CPU caches.  (There are tricks

            
            to avoid this copying by playing with the page tables, but they're unpleasantly

            
            complicated and turn out to be about as expensive as the copying anyway.)

            
            More to the point, all the work ramfs is doing has to happen _anyway_,

            
            since all file access goes through the page and dentry caches.  The RAM

            
            disk is simply unnecessary; ramfs is internally much simpler.


            Another reason ramdisks are semi-obsolete is that the introduction of

            
            loopback devices offered a more flexible and convenient way to create

            
            synthetic block devices, now from files instead of from chunks of memory.

            
            See losetup (8) for details.


            ramfs and tmpfs:

            
            ----------------


            One downside of ramfs is you can keep writing data into it until you fill

            
            up all memory, and the VM can't free it because the VM thinks that files

            
            should get written to backing store (rather than swap space), but ramfs hasn't

            
            got any backing store.  Because of this, only root (or a trusted user) should

            
            be allowed write access to a ramfs mount.


            A ramfs derivative called tmpfs was created to add size limits, and the ability

            
            to write the data to swap space.  Normal users can be allowed write access to

            
            tmpfs mounts.  See Documentation/filesystems/tmpfs.txt for more information.


            What is rootfs?

            
            ---------------


            Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is

            
            always present in 2.6 systems.  You can't unmount rootfs for approximately the

            
            same reason you can't kill the init process; rather than having special code

            
            to check for and handle an empty list, it's smaller and simpler for the kernel

            
            to just make sure certain lists can't become empty.


            Most systems just mount another filesystem over rootfs and ignore it.  The

            
            amount of space an empty instance of ramfs takes up is tiny.


            If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by

            
            default.  To force ramfs, add "rootfstype=ramfs" to the kernel command

            
            line.


            What is initramfs?

            
            ------------------


            All 2.6 Linux kernels contain a gzipped "cpio" format archive, which is

            
            extracted into rootfs when the kernel boots up.  After extracting, the kernel

            
            checks to see if rootfs contains a file "init", and if so it executes it as PID

            
            1.  If found, this init process is responsible for bringing the system the

            
            rest of the way up, including locating and mounting the real root device (if

            
            any).  If rootfs does not contain an init program after the embedded cpio

            
            archive is extracted into it, the kernel will fall through to the older code

            
            to locate and mount a root partition, then exec some variant of /sbin/init

            
            out of that.


            All this differs from the old initrd in several ways:


              - The old initrd was always a separate file, while the initramfs archive is

            
                linked into the linux kernel image.  (The directory linux-*/usr is devoted

            
                to generating this archive during the build.)


              - The old initrd file was a gzipped filesystem image (in some file format,

            
                such as ext2, that needed a driver built into the kernel), while the new

            
                initramfs archive is a gzipped cpio archive (like tar only simpler,

            
                see cpio(1) and Documentation/early-userspace/buffer-format.txt).  The

            
                kernel's cpio extraction code is not only extremely small, it's also

            
                __init text and data that can be discarded during the boot process.


              - The program run by the old initrd (which was called /initrd, not /init) did

            
                some setup and then returned to the kernel, while the init program from

            
                initramfs is not expected to return to the kernel.  (If /init needs to hand

            
                off control it can overmount / with a new root device and exec another init

            
                program.  See the switch_root utility, below.)


              - When switching another root device, initrd would pivot_root and then

            
                umount the ramdisk.  But initramfs is rootfs: you can neither pivot_root

            
                rootfs, nor unmount it.  Instead delete everything out of rootfs to

            
                free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs

            
                with the new root (cd /newmount; mount --move . /; chroot .), attach

            
                stdin/stdout/stderr to the new /dev/console, and exec the new init.


                Since this is a remarkably persnickety process (and involves deleting

            
                commands before you can run them), the klibc package introduced a helper

            
                program (utils/run_init.c) to do all this for you.  Most other packages

            
                (such as busybox) have named this command "switch_root".


            Populating initramfs:

            
            ---------------------


            The 2.6 kernel build process always creates a gzipped cpio format initramfs

            
            archive and links it into the resulting kernel binary.  By default, this

            
            archive is empty (consuming 134 bytes on x86).


            The config option CONFIG_INITRAMFS_SOURCE (in General Setup in menuconfig,

            
            and living in usr/Kconfig) can be used to specify a source for the

            
            initramfs archive, which will automatically be incorporated into the

            
            resulting binary.  This option can point to an existing gzipped cpio

            
            archive, a directory containing files to be archived, or a text file

            
            specification such as the following example:


              dir /dev 755 0 0

            
              nod /dev/console 644 0 0 c 5 1

            
              nod /dev/loop0 644 0 0 b 7 0

            
              dir /bin 755 1000 1000

            
              slink /bin/sh busybox 777 0 0

            
              file /bin/busybox initramfs/busybox 755 0 0

            
              dir /proc 755 0 0

            
              dir /sys 755 0 0

            
              dir /mnt 755 0 0

            
              file /init initramfs/init.sh 755 0 0


            Run "usr/gen_init_cpio" (after the kernel build) to get a usage message

            
            documenting the above file format.


            One advantage of the configuration file is that root access is not required to

            
            set permissions or create device nodes in the new archive.  (Note that those

            
            two example "file" entries expect to find files named "init.sh" and "busybox" in

            
            a directory called "initramfs", under the linux-2.6.* directory.  See

            
            Documentation/early-userspace/README for more details.)


            The kernel does not depend on external cpio tools.  If you specify a

            
            directory instead of a configuration file, the kernel's build infrastructure

            
            creates a configuration file from that directory (usr/Makefile calls

            
            scripts/gen_initramfs_list.sh), and proceeds to package up that directory

            
            using the config file (by feeding it to usr/gen_init_cpio, which is created

            
            from usr/gen_init_cpio.c).  The kernel's build-time cpio creation code is

            
            entirely self-contained, and the kernel's boot-time extractor is also

            
            (obviously) self-contained.


            The one thing you might need external cpio utilities installed for is creating

            
            or extracting your own preprepared cpio files to feed to the kernel build

            
            (instead of a config file or directory).


            The following command line can extract a cpio image (either by the above script

            
            or by the kernel build) back into its component files:


              cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames


            The following shell script can create a prebuilt cpio archive you can

            
            use in place of the above config file:


              #!/bin/sh


              # Copyright 2006 Rob Landley <rob@landley.net> and TimeSys Corporation.

            
              # Licensed under GPL version 2


              if [ $# -ne 2 ]

            
              then

            
                echo "usage: mkinitramfs directory imagename.cpio.gz"

            
                exit 1

            
              fi


              if [ -d "$1" ]

            
              then

            
                echo "creating $2 from $1"

            
                (cd "$1"; find . | cpio -o -H newc | gzip) > "$2"

            
              else

            
                echo "First argument must be a directory"

            
                exit 1

            
              fi


            Note: The cpio man page contains some bad advice that will break your initramfs

            
            archive if you follow it.  It says "A typical way to generate the list

            
            of filenames is with the find command; you should give find the -depth option

            
            to minimize problems with permissions on directories that are unwritable or not

            
            searchable."  Don't do this when creating initramfs.cpio.gz images, it won't

            
            work.  The Linux kernel cpio extractor won't create files in a directory that

            
            doesn't exist, so the directory entries must go before the files that go in

            
            those directories.  The above script gets them in the right order.


            External initramfs images:

            
            --------------------------


            If the kernel has initrd support enabled, an external cpio.gz archive can also

            
            be passed into a 2.6 kernel in place of an initrd.  In this case, the kernel

            
            will autodetect the type (initramfs, not initrd) and extract the external cpio

            
            archive into rootfs before trying to run /init.


            This has the memory efficiency advantages of initramfs (no ramdisk block

            
            device) but the separate packaging of initrd (which is nice if you have

            
            non-GPL code you'd like to run from initramfs, without conflating it with

            
            the GPL licensed Linux kernel binary).


            It can also be used to supplement the kernel's built-in initramfs image.  The

            
            files in the external archive will overwrite any conflicting files in

            
            the built-in initramfs archive.  Some distributors also prefer to customize

            
            a single kernel image with task-specific initramfs images, without recompiling.


            Contents of initramfs:

            
            ----------------------


            An initramfs archive is a complete self-contained root filesystem for Linux.

            
            If you don't already understand what shared libraries, devices, and paths

            
            you need to get a minimal root filesystem up and running, here are some

            
            references:

            
            http://www.tldp.org/HOWTO/Bootdisk-HOWTO/

            
            http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html

            
            http://www.linuxfromscratch.org/lfs/view/stable/


            The "klibc" package (http://www.kernel.org/pub/linux/libs/klibc) is

            
            designed to be a tiny C library to statically link early userspace

            
            code against, along with some related utilities.  It is BSD licensed.


            I use uClibc (http://www.uclibc.org) and busybox (http://www.busybox.net)

            
            myself.  These are LGPL and GPL, respectively.  (A self-contained initramfs

            
            package is planned for the busybox 1.3 release.)


            In theory you could use glibc, but that's not well suited for small embedded

            
            uses like this.  (A "hello world" program statically linked against glibc is

            
            over 400k.  With uClibc it's 7k.  Also note that glibc dlopens libnss to do

            
            name lookups, even when otherwise statically linked.)


            A good first step is to get initramfs to run a statically linked "hello world"

            
            program as init, and test it under an emulator like qemu (www.qemu.org) or

            
            User Mode Linux, like so:


              cat > hello.c << EOF

            
              #include <stdio.h>

            
              #include <unistd.h>


              int main(int argc, char *argv[])

            
              {

            
                printf("Hello world!\n");

            
                sleep(999999999);

            
              }

            
              EOF

            
              gcc -static hello.c -o init

            
              echo init | cpio -o -H newc | gzip > test.cpio.gz

            
              # Testing external initramfs using the initrd loading mechanism.

            
              qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero


            When debugging a normal root filesystem, it's nice to be able to boot with

            
            "init=/bin/sh".  The initramfs equivalent is "rdinit=/bin/sh", and it's

            
            just as useful.


            Why cpio rather than tar?

            
            -------------------------


            This decision was made back in December, 2001.  The discussion started here:


              http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html


            And spawned a second thread (specifically on tar vs cpio), starting here:


              http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html


            The quick and dirty summary version (which is no substitute for reading

            
            the above threads) is:


            1) cpio is a standard.  It's decades old (from the AT&T days), and already

            
               widely used on Linux (inside RPM, Red Hat's device driver disks).  Here's

            
               a Linux Journal article about it from 1996:


                  http://www.linuxjournal.com/article/1213


               It's not as popular as tar because the traditional cpio command line tools

            
               require _truly_hideous_ command line arguments.  But that says nothing

            
               either way about the archive format, and there are alternative tools,

            
               such as:


                 http://freecode.com/projects/afio


            2) The cpio archive format chosen by the kernel is simpler and cleaner (and

            
               thus easier to create and parse) than any of the (literally dozens of)

            
               various tar archive formats.  The complete initramfs archive format is

            
               explained in buffer-format.txt, created in usr/gen_init_cpio.c, and

            
               extracted in init/initramfs.c.  All three together come to less than 26k

            
               total of human-readable text.


            3) The GNU project standardizing on tar is approximately as relevant as

            
               Windows standardizing on zip.  Linux is not part of either, and is free

            
               to make its own technical decisions.


            4) Since this is a kernel internal format, it could easily have been

            
               something brand new.  The kernel provides its own tools to create and

            
               extract this format anyway.  Using an existing standard was preferable,

            
               but not essential.


            5) Al Viro made the decision (quote: "tar is ugly as hell and not going to be

            
               supported on the kernel side"):


                  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html


               explained his reasoning:


                  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html

            
                  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html


               and, most importantly, designed and implemented the initramfs code.


            Future directions:

            
            ------------------


            Today (2.6.16), initramfs is always compiled in, but not always used.  The

            
            kernel falls back to legacy boot code that is reached only if initramfs does

            
            not contain an /init program.  The fallback is legacy code, there to ensure a

            
            smooth transition and allowing early boot functionality to gradually move to

            
            "early userspace" (I.E. initramfs).


            The move to early userspace is necessary because finding and mounting the real

            
            root device is complex.  Root partitions can span multiple devices (raid or

            
            separate journal).  They can be out on the network (requiring dhcp, setting a

            
            specific MAC address, logging into a server, etc).  They can live on removable

            
            media, with dynamically allocated major/minor numbers and persistent naming

            
            issues requiring a full udev implementation to sort out.  They can be

            
            compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,

            
            and so on.


            This kind of complexity (which inevitably includes policy) is rightly handled

            
            in userspace.  Both klibc and busybox/uClibc are working on simple initramfs

            
            packages to drop into a kernel build.


            The klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree.

            
            The kernel's current early boot code (partition detection, etc) will probably

            
            be migrated into a default initramfs, automatically created and used by the

            
            kernel build.
	ramfs, rootfs and initramfs
	October 17, 2005
	Rob Landley <rob@landley.net>
	=============================

	What is ramfs?
	--------------

	Ramfs is a very simple filesystem that exports Linux's disk caching
	mechanisms (the page cache and dentry cache) as a dynamically resizable
	RAM-based filesystem.

	Normally all files are cached in memory by Linux. Pages of data read from
	backing store (usually the block device the filesystem is mounted on) are kept
	around in case it's needed again, but marked as clean (freeable) in case the
	Virtual Memory system needs the memory for something else. Similarly, data
	written to files is marked clean as soon as it has been written to backing
	store, but kept around for caching purposes until the VM reallocates the
	memory. A similar mechanism (the dentry cache) greatly speeds up access to
	directories.

	With ramfs, there is no backing store. Files written into ramfs allocate
	dentries and page cache as usual, but there's nowhere to write them to.
	This means the pages are never marked clean, so they can't be freed by the
	VM when it's looking to recycle memory.

	The amount of code required to implement ramfs is tiny, because all the
	work is done by the existing Linux caching infrastructure. Basically,
	you're mounting the disk cache as a filesystem. Because of this, ramfs is not
	an optional component removable via menuconfig, since there would be negligible
	space savings.

	ramfs and ramdisk:
	------------------

	The older "ram disk" mechanism created a synthetic block device out of
	an area of RAM and used it as backing store for a filesystem. This block
	device was of fixed size, so the filesystem mounted on it was of fixed
	size. Using a ram disk also required unnecessarily copying memory from the
	fake block device into the page cache (and copying changes back out), as well
	as creating and destroying dentries. Plus it needed a filesystem driver
	(such as ext2) to format and interpret this data.

	Compared to ramfs, this wastes memory (and memory bus bandwidth), creates
	unnecessary work for the CPU, and pollutes the CPU caches. (There are tricks
	to avoid this copying by playing with the page tables, but they're unpleasantly
	complicated and turn out to be about as expensive as the copying anyway.)
	More to the point, all the work ramfs is doing has to happen _anyway_,
	since all file access goes through the page and dentry caches. The RAM
	disk is simply unnecessary; ramfs is internally much simpler.

	Another reason ramdisks are semi-obsolete is that the introduction of
	loopback devices offered a more flexible and convenient way to create
	synthetic block devices, now from files instead of from chunks of memory.
	See losetup (8) for details.

	ramfs and tmpfs:
	----------------

	One downside of ramfs is you can keep writing data into it until you fill
	up all memory, and the VM can't free it because the VM thinks that files
	should get written to backing store (rather than swap space), but ramfs hasn't
	got any backing store. Because of this, only root (or a trusted user) should
	be allowed write access to a ramfs mount.

	A ramfs derivative called tmpfs was created to add size limits, and the ability
	to write the data to swap space. Normal users can be allowed write access to
	tmpfs mounts. See Documentation/filesystems/tmpfs.txt for more information.

	What is rootfs?
	---------------

	Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is
	always present in 2.6 systems. You can't unmount rootfs for approximately the
	same reason you can't kill the init process; rather than having special code
	to check for and handle an empty list, it's smaller and simpler for the kernel
	to just make sure certain lists can't become empty.

	Most systems just mount another filesystem over rootfs and ignore it. The
	amount of space an empty instance of ramfs takes up is tiny.

	If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
	default. To force ramfs, add "rootfstype=ramfs" to the kernel command
	line.

	What is initramfs?
	------------------

	All 2.6 Linux kernels contain a gzipped "cpio" format archive, which is
	extracted into rootfs when the kernel boots up. After extracting, the kernel
	checks to see if rootfs contains a file "init", and if so it executes it as PID
	1. If found, this init process is responsible for bringing the system the
	rest of the way up, including locating and mounting the real root device (if
	any). If rootfs does not contain an init program after the embedded cpio
	archive is extracted into it, the kernel will fall through to the older code
	to locate and mount a root partition, then exec some variant of /sbin/init
	out of that.

	All this differs from the old initrd in several ways:

	- The old initrd was always a separate file, while the initramfs archive is
	linked into the linux kernel image. (The directory linux-*/usr is devoted
	to generating this archive during the build.)

	- The old initrd file was a gzipped filesystem image (in some file format,
	such as ext2, that needed a driver built into the kernel), while the new
	initramfs archive is a gzipped cpio archive (like tar only simpler,
	see cpio(1) and Documentation/early-userspace/buffer-format.txt). The
	kernel's cpio extraction code is not only extremely small, it's also
	__init text and data that can be discarded during the boot process.

	- The program run by the old initrd (which was called /initrd, not /init) did
	some setup and then returned to the kernel, while the init program from
	initramfs is not expected to return to the kernel. (If /init needs to hand
	off control it can overmount / with a new root device and exec another init
	program. See the switch_root utility, below.)

	- When switching another root device, initrd would pivot_root and then
	umount the ramdisk. But initramfs is rootfs: you can neither pivot_root
	rootfs, nor unmount it. Instead delete everything out of rootfs to
	free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
	with the new root (cd /newmount; mount --move . /; chroot .), attach
	stdin/stdout/stderr to the new /dev/console, and exec the new init.

	Since this is a remarkably persnickety process (and involves deleting
	commands before you can run them), the klibc package introduced a helper
	program (utils/run_init.c) to do all this for you. Most other packages
	(such as busybox) have named this command "switch_root".

	Populating initramfs:
	---------------------

	The 2.6 kernel build process always creates a gzipped cpio format initramfs
	archive and links it into the resulting kernel binary. By default, this
	archive is empty (consuming 134 bytes on x86).

	The config option CONFIG_INITRAMFS_SOURCE (in General Setup in menuconfig,
	and living in usr/Kconfig) can be used to specify a source for the
	initramfs archive, which will automatically be incorporated into the
	resulting binary. This option can point to an existing gzipped cpio
	archive, a directory containing files to be archived, or a text file
	specification such as the following example:

	dir /dev 755 0 0
	nod /dev/console 644 0 0 c 5 1
	nod /dev/loop0 644 0 0 b 7 0
	dir /bin 755 1000 1000
	slink /bin/sh busybox 777 0 0
	file /bin/busybox initramfs/busybox 755 0 0
	dir /proc 755 0 0
	dir /sys 755 0 0
	dir /mnt 755 0 0
	file /init initramfs/init.sh 755 0 0

	Run "usr/gen_init_cpio" (after the kernel build) to get a usage message
	documenting the above file format.

	One advantage of the configuration file is that root access is not required to
	set permissions or create device nodes in the new archive. (Note that those
	two example "file" entries expect to find files named "init.sh" and "busybox" in
	a directory called "initramfs", under the linux-2.6.* directory. See
	Documentation/early-userspace/README for more details.)

	The kernel does not depend on external cpio tools. If you specify a
	directory instead of a configuration file, the kernel's build infrastructure
	creates a configuration file from that directory (usr/Makefile calls
	scripts/gen_initramfs_list.sh), and proceeds to package up that directory
	using the config file (by feeding it to usr/gen_init_cpio, which is created
	from usr/gen_init_cpio.c). The kernel's build-time cpio creation code is
	entirely self-contained, and the kernel's boot-time extractor is also
	(obviously) self-contained.

	The one thing you might need external cpio utilities installed for is creating
	or extracting your own preprepared cpio files to feed to the kernel build
	(instead of a config file or directory).

	The following command line can extract a cpio image (either by the above script
	or by the kernel build) back into its component files:

	cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames

	The following shell script can create a prebuilt cpio archive you can
	use in place of the above config file:

	#!/bin/sh

	# Copyright 2006 Rob Landley <rob@landley.net> and TimeSys Corporation.
	# Licensed under GPL version 2

	if [ $# -ne 2 ]
	then
	echo "usage: mkinitramfs directory imagename.cpio.gz"
	exit 1
	fi

	if [ -d "$1" ]
	then
	echo "creating $2 from $1"
	(cd "$1"; find . \| cpio -o -H newc \| gzip) > "$2"
	else
	echo "First argument must be a directory"
	exit 1
	fi

	Note: The cpio man page contains some bad advice that will break your initramfs
	archive if you follow it. It says "A typical way to generate the list
	of filenames is with the find command; you should give find the -depth option
	to minimize problems with permissions on directories that are unwritable or not
	searchable." Don't do this when creating initramfs.cpio.gz images, it won't
	work. The Linux kernel cpio extractor won't create files in a directory that
	doesn't exist, so the directory entries must go before the files that go in
	those directories. The above script gets them in the right order.

	External initramfs images:
	--------------------------

	If the kernel has initrd support enabled, an external cpio.gz archive can also
	be passed into a 2.6 kernel in place of an initrd. In this case, the kernel
	will autodetect the type (initramfs, not initrd) and extract the external cpio
	archive into rootfs before trying to run /init.

	This has the memory efficiency advantages of initramfs (no ramdisk block
	device) but the separate packaging of initrd (which is nice if you have
	non-GPL code you'd like to run from initramfs, without conflating it with
	the GPL licensed Linux kernel binary).

	It can also be used to supplement the kernel's built-in initramfs image. The
	files in the external archive will overwrite any conflicting files in
	the built-in initramfs archive. Some distributors also prefer to customize
	a single kernel image with task-specific initramfs images, without recompiling.

	Contents of initramfs:
	----------------------

	An initramfs archive is a complete self-contained root filesystem for Linux.
	If you don't already understand what shared libraries, devices, and paths
	you need to get a minimal root filesystem up and running, here are some
	references:
	http://www.tldp.org/HOWTO/Bootdisk-HOWTO/
	http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html
	http://www.linuxfromscratch.org/lfs/view/stable/

	The "klibc" package (http://www.kernel.org/pub/linux/libs/klibc) is
	designed to be a tiny C library to statically link early userspace
	code against, along with some related utilities. It is BSD licensed.

	I use uClibc (http://www.uclibc.org) and busybox (http://www.busybox.net)
	myself. These are LGPL and GPL, respectively. (A self-contained initramfs
	package is planned for the busybox 1.3 release.)

	In theory you could use glibc, but that's not well suited for small embedded
	uses like this. (A "hello world" program statically linked against glibc is
	over 400k. With uClibc it's 7k. Also note that glibc dlopens libnss to do
	name lookups, even when otherwise statically linked.)

	A good first step is to get initramfs to run a statically linked "hello world"
	program as init, and test it under an emulator like qemu (www.qemu.org) or
	User Mode Linux, like so:

	cat > hello.c << EOF
	#include <stdio.h>
	#include <unistd.h>

	int main(int argc, char *argv[])
	{
	printf("Hello world!\n");
	sleep(999999999);
	}
	EOF
	gcc -static hello.c -o init
	echo init \| cpio -o -H newc \| gzip > test.cpio.gz
	# Testing external initramfs using the initrd loading mechanism.
	qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero

	When debugging a normal root filesystem, it's nice to be able to boot with
	"init=/bin/sh". The initramfs equivalent is "rdinit=/bin/sh", and it's
	just as useful.

	Why cpio rather than tar?
	-------------------------

	This decision was made back in December, 2001. The discussion started here:

	http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html

	And spawned a second thread (specifically on tar vs cpio), starting here:

	http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html

	The quick and dirty summary version (which is no substitute for reading
	the above threads) is:

	1) cpio is a standard. It's decades old (from the AT&T days), and already
	widely used on Linux (inside RPM, Red Hat's device driver disks). Here's
	a Linux Journal article about it from 1996:

	http://www.linuxjournal.com/article/1213

	It's not as popular as tar because the traditional cpio command line tools
	require _truly_hideous_ command line arguments. But that says nothing
	either way about the archive format, and there are alternative tools,
	such as:

	http://freecode.com/projects/afio

	2) The cpio archive format chosen by the kernel is simpler and cleaner (and
	thus easier to create and parse) than any of the (literally dozens of)
	various tar archive formats. The complete initramfs archive format is
	explained in buffer-format.txt, created in usr/gen_init_cpio.c, and
	extracted in init/initramfs.c. All three together come to less than 26k
	total of human-readable text.

	3) The GNU project standardizing on tar is approximately as relevant as
	Windows standardizing on zip. Linux is not part of either, and is free
	to make its own technical decisions.

	4) Since this is a kernel internal format, it could easily have been
	something brand new. The kernel provides its own tools to create and
	extract this format anyway. Using an existing standard was preferable,
	but not essential.

	5) Al Viro made the decision (quote: "tar is ugly as hell and not going to be
	supported on the kernel side"):

	http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html

	explained his reasoning:

	http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html
	http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html

	and, most importantly, designed and implemented the initramfs code.

	Future directions:
	------------------

	Today (2.6.16), initramfs is always compiled in, but not always used. The
	kernel falls back to legacy boot code that is reached only if initramfs does
	not contain an /init program. The fallback is legacy code, there to ensure a
	smooth transition and allowing early boot functionality to gradually move to
	"early userspace" (I.E. initramfs).

	The move to early userspace is necessary because finding and mounting the real
	root device is complex. Root partitions can span multiple devices (raid or
	separate journal). They can be out on the network (requiring dhcp, setting a
	specific MAC address, logging into a server, etc). They can live on removable
	media, with dynamically allocated major/minor numbers and persistent naming
	issues requiring a full udev implementation to sort out. They can be
	compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
	and so on.

	This kind of complexity (which inevitably includes policy) is rightly handled
	in userspace. Both klibc and busybox/uClibc are working on simple initramfs
	packages to drop into a kernel build.

	The klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree.
	The kernel's current early boot code (partition detection, etc) will probably
	be migrated into a default initramfs, automatically created and used by the
	kernel build.