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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

1	ramfs, rootfs and initramfs
2	October 17, 2005
3	Rob Landley <rob@landley.net>
4	=============================
5	
6	What is ramfs?
7	--------------
8	
9	Ramfs is a very simple filesystem that exports Linux's disk caching
10	mechanisms (the page cache and dentry cache) as a dynamically resizable
11	RAM-based filesystem.
12	
13	Normally all files are cached in memory by Linux.  Pages of data read from
14	backing store (usually the block device the filesystem is mounted on) are kept
15	around in case it's needed again, but marked as clean (freeable) in case the
16	Virtual Memory system needs the memory for something else.  Similarly, data
17	written to files is marked clean as soon as it has been written to backing
18	store, but kept around for caching purposes until the VM reallocates the
19	memory.  A similar mechanism (the dentry cache) greatly speeds up access to
20	directories.
21	
22	With ramfs, there is no backing store.  Files written into ramfs allocate
23	dentries and page cache as usual, but there's nowhere to write them to.
24	This means the pages are never marked clean, so they can't be freed by the
25	VM when it's looking to recycle memory.
26	
27	The amount of code required to implement ramfs is tiny, because all the
28	work is done by the existing Linux caching infrastructure.  Basically,
29	you're mounting the disk cache as a filesystem.  Because of this, ramfs is not
30	an optional component removable via menuconfig, since there would be negligible
31	space savings.
32	
33	ramfs and ramdisk:
34	------------------
35	
36	The older "ram disk" mechanism created a synthetic block device out of
37	an area of RAM and used it as backing store for a filesystem.  This block
38	device was of fixed size, so the filesystem mounted on it was of fixed
39	size.  Using a ram disk also required unnecessarily copying memory from the
40	fake block device into the page cache (and copying changes back out), as well
41	as creating and destroying dentries.  Plus it needed a filesystem driver
42	(such as ext2) to format and interpret this data.
43	
44	Compared to ramfs, this wastes memory (and memory bus bandwidth), creates
45	unnecessary work for the CPU, and pollutes the CPU caches.  (There are tricks
46	to avoid this copying by playing with the page tables, but they're unpleasantly
47	complicated and turn out to be about as expensive as the copying anyway.)
48	More to the point, all the work ramfs is doing has to happen _anyway_,
49	since all file access goes through the page and dentry caches.  The RAM
50	disk is simply unnecessary; ramfs is internally much simpler.
51	
52	Another reason ramdisks are semi-obsolete is that the introduction of
53	loopback devices offered a more flexible and convenient way to create
54	synthetic block devices, now from files instead of from chunks of memory.
55	See losetup (8) for details.
56	
57	ramfs and tmpfs:
58	----------------
59	
60	One downside of ramfs is you can keep writing data into it until you fill
61	up all memory, and the VM can't free it because the VM thinks that files
62	should get written to backing store (rather than swap space), but ramfs hasn't
63	got any backing store.  Because of this, only root (or a trusted user) should
64	be allowed write access to a ramfs mount.
65	
66	A ramfs derivative called tmpfs was created to add size limits, and the ability
67	to write the data to swap space.  Normal users can be allowed write access to
68	tmpfs mounts.  See Documentation/filesystems/tmpfs.txt for more information.
69	
70	What is rootfs?
71	---------------
72	
73	Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is
74	always present in 2.6 systems.  You can't unmount rootfs for approximately the
75	same reason you can't kill the init process; rather than having special code
76	to check for and handle an empty list, it's smaller and simpler for the kernel
77	to just make sure certain lists can't become empty.
78	
79	Most systems just mount another filesystem over rootfs and ignore it.  The
80	amount of space an empty instance of ramfs takes up is tiny.
81	
82	If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
83	default.  To force ramfs, add "rootfstype=ramfs" to the kernel command
84	line.
85	
86	What is initramfs?
87	------------------
88	
89	All 2.6 Linux kernels contain a gzipped "cpio" format archive, which is
90	extracted into rootfs when the kernel boots up.  After extracting, the kernel
91	checks to see if rootfs contains a file "init", and if so it executes it as PID
92	1.  If found, this init process is responsible for bringing the system the
93	rest of the way up, including locating and mounting the real root device (if
94	any).  If rootfs does not contain an init program after the embedded cpio
95	archive is extracted into it, the kernel will fall through to the older code
96	to locate and mount a root partition, then exec some variant of /sbin/init
97	out of that.
98	
99	All this differs from the old initrd in several ways:
100	
101	  - The old initrd was always a separate file, while the initramfs archive is
102	    linked into the linux kernel image.  (The directory linux-*/usr is devoted
103	    to generating this archive during the build.)
104	
105	  - The old initrd file was a gzipped filesystem image (in some file format,
106	    such as ext2, that needed a driver built into the kernel), while the new
107	    initramfs archive is a gzipped cpio archive (like tar only simpler,
108	    see cpio(1) and Documentation/early-userspace/buffer-format.txt).  The
109	    kernel's cpio extraction code is not only extremely small, it's also
110	    __init text and data that can be discarded during the boot process.
111	
112	  - The program run by the old initrd (which was called /initrd, not /init) did
113	    some setup and then returned to the kernel, while the init program from
114	    initramfs is not expected to return to the kernel.  (If /init needs to hand
115	    off control it can overmount / with a new root device and exec another init
116	    program.  See the switch_root utility, below.)
117	
118	  - When switching another root device, initrd would pivot_root and then
119	    umount the ramdisk.  But initramfs is rootfs: you can neither pivot_root
120	    rootfs, nor unmount it.  Instead delete everything out of rootfs to
121	    free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
122	    with the new root (cd /newmount; mount --move . /; chroot .), attach
123	    stdin/stdout/stderr to the new /dev/console, and exec the new init.
124	
125	    Since this is a remarkably persnickety process (and involves deleting
126	    commands before you can run them), the klibc package introduced a helper
127	    program (utils/run_init.c) to do all this for you.  Most other packages
128	    (such as busybox) have named this command "switch_root".
129	
130	Populating initramfs:
131	---------------------
132	
133	The 2.6 kernel build process always creates a gzipped cpio format initramfs
134	archive and links it into the resulting kernel binary.  By default, this
135	archive is empty (consuming 134 bytes on x86).
136	
137	The config option CONFIG_INITRAMFS_SOURCE (in General Setup in menuconfig,
138	and living in usr/Kconfig) can be used to specify a source for the
139	initramfs archive, which will automatically be incorporated into the
140	resulting binary.  This option can point to an existing gzipped cpio
141	archive, a directory containing files to be archived, or a text file
142	specification such as the following example:
143	
144	  dir /dev 755 0 0
145	  nod /dev/console 644 0 0 c 5 1
146	  nod /dev/loop0 644 0 0 b 7 0
147	  dir /bin 755 1000 1000
148	  slink /bin/sh busybox 777 0 0
149	  file /bin/busybox initramfs/busybox 755 0 0
150	  dir /proc 755 0 0
151	  dir /sys 755 0 0
152	  dir /mnt 755 0 0
153	  file /init initramfs/init.sh 755 0 0
154	
155	Run "usr/gen_init_cpio" (after the kernel build) to get a usage message
156	documenting the above file format.
157	
158	One advantage of the configuration file is that root access is not required to
159	set permissions or create device nodes in the new archive.  (Note that those
160	two example "file" entries expect to find files named "init.sh" and "busybox" in
161	a directory called "initramfs", under the linux-2.6.* directory.  See
162	Documentation/early-userspace/README for more details.)
163	
164	The kernel does not depend on external cpio tools.  If you specify a
165	directory instead of a configuration file, the kernel's build infrastructure
166	creates a configuration file from that directory (usr/Makefile calls
167	scripts/gen_initramfs_list.sh), and proceeds to package up that directory
168	using the config file (by feeding it to usr/gen_init_cpio, which is created
169	from usr/gen_init_cpio.c).  The kernel's build-time cpio creation code is
170	entirely self-contained, and the kernel's boot-time extractor is also
171	(obviously) self-contained.
172	
173	The one thing you might need external cpio utilities installed for is creating
174	or extracting your own preprepared cpio files to feed to the kernel build
175	(instead of a config file or directory).
176	
177	The following command line can extract a cpio image (either by the above script
178	or by the kernel build) back into its component files:
179	
180	  cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames
181	
182	The following shell script can create a prebuilt cpio archive you can
183	use in place of the above config file:
184	
185	  #!/bin/sh
186	
187	  # Copyright 2006 Rob Landley <rob@landley.net> and TimeSys Corporation.
188	  # Licensed under GPL version 2
189	
190	  if [ $# -ne 2 ]
191	  then
192	    echo "usage: mkinitramfs directory imagename.cpio.gz"
193	    exit 1
194	  fi
195	
196	  if [ -d "$1" ]
197	  then
198	    echo "creating $2 from $1"
199	    (cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
200	  else
201	    echo "First argument must be a directory"
202	    exit 1
203	  fi
204	
205	Note: The cpio man page contains some bad advice that will break your initramfs
206	archive if you follow it.  It says "A typical way to generate the list
207	of filenames is with the find command; you should give find the -depth option
208	to minimize problems with permissions on directories that are unwritable or not
209	searchable."  Don't do this when creating initramfs.cpio.gz images, it won't
210	work.  The Linux kernel cpio extractor won't create files in a directory that
211	doesn't exist, so the directory entries must go before the files that go in
212	those directories.  The above script gets them in the right order.
213	
214	External initramfs images:
215	--------------------------
216	
217	If the kernel has initrd support enabled, an external cpio.gz archive can also
218	be passed into a 2.6 kernel in place of an initrd.  In this case, the kernel
219	will autodetect the type (initramfs, not initrd) and extract the external cpio
220	archive into rootfs before trying to run /init.
221	
222	This has the memory efficiency advantages of initramfs (no ramdisk block
223	device) but the separate packaging of initrd (which is nice if you have
224	non-GPL code you'd like to run from initramfs, without conflating it with
225	the GPL licensed Linux kernel binary).
226	
227	It can also be used to supplement the kernel's built-in initramfs image.  The
228	files in the external archive will overwrite any conflicting files in
229	the built-in initramfs archive.  Some distributors also prefer to customize
230	a single kernel image with task-specific initramfs images, without recompiling.
231	
232	Contents of initramfs:
233	----------------------
234	
235	An initramfs archive is a complete self-contained root filesystem for Linux.
236	If you don't already understand what shared libraries, devices, and paths
237	you need to get a minimal root filesystem up and running, here are some
238	references:
239	http://www.tldp.org/HOWTO/Bootdisk-HOWTO/
240	http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html
241	http://www.linuxfromscratch.org/lfs/view/stable/
242	
243	The "klibc" package (http://www.kernel.org/pub/linux/libs/klibc) is
244	designed to be a tiny C library to statically link early userspace
245	code against, along with some related utilities.  It is BSD licensed.
246	
247	I use uClibc (http://www.uclibc.org) and busybox (http://www.busybox.net)
248	myself.  These are LGPL and GPL, respectively.  (A self-contained initramfs
249	package is planned for the busybox 1.3 release.)
250	
251	In theory you could use glibc, but that's not well suited for small embedded
252	uses like this.  (A "hello world" program statically linked against glibc is
253	over 400k.  With uClibc it's 7k.  Also note that glibc dlopens libnss to do
254	name lookups, even when otherwise statically linked.)
255	
256	A good first step is to get initramfs to run a statically linked "hello world"
257	program as init, and test it under an emulator like qemu (www.qemu.org) or
258	User Mode Linux, like so:
259	
260	  cat > hello.c << EOF
261	  #include <stdio.h>
262	  #include <unistd.h>
263	
264	  int main(int argc, char *argv[])
265	  {
266	    printf("Hello world!\n");
267	    sleep(999999999);
268	  }
269	  EOF
270	  gcc -static hello.c -o init
271	  echo init | cpio -o -H newc | gzip > test.cpio.gz
272	  # Testing external initramfs using the initrd loading mechanism.
273	  qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero
274	
275	When debugging a normal root filesystem, it's nice to be able to boot with
276	"init=/bin/sh".  The initramfs equivalent is "rdinit=/bin/sh", and it's
277	just as useful.
278	
279	Why cpio rather than tar?
280	-------------------------
281	
282	This decision was made back in December, 2001.  The discussion started here:
283	
284	  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html
285	
286	And spawned a second thread (specifically on tar vs cpio), starting here:
287	
288	  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html
289	
290	The quick and dirty summary version (which is no substitute for reading
291	the above threads) is:
292	
293	1) cpio is a standard.  It's decades old (from the AT&T days), and already
294	   widely used on Linux (inside RPM, Red Hat's device driver disks).  Here's
295	   a Linux Journal article about it from 1996:
296	
297	      http://www.linuxjournal.com/article/1213
298	
299	   It's not as popular as tar because the traditional cpio command line tools
300	   require _truly_hideous_ command line arguments.  But that says nothing
301	   either way about the archive format, and there are alternative tools,
302	   such as:
303	
304	     http://freecode.com/projects/afio
305	
306	2) The cpio archive format chosen by the kernel is simpler and cleaner (and
307	   thus easier to create and parse) than any of the (literally dozens of)
308	   various tar archive formats.  The complete initramfs archive format is
309	   explained in buffer-format.txt, created in usr/gen_init_cpio.c, and
310	   extracted in init/initramfs.c.  All three together come to less than 26k
311	   total of human-readable text.
312	
313	3) The GNU project standardizing on tar is approximately as relevant as
314	   Windows standardizing on zip.  Linux is not part of either, and is free
315	   to make its own technical decisions.
316	
317	4) Since this is a kernel internal format, it could easily have been
318	   something brand new.  The kernel provides its own tools to create and
319	   extract this format anyway.  Using an existing standard was preferable,
320	   but not essential.
321	
322	5) Al Viro made the decision (quote: "tar is ugly as hell and not going to be
323	   supported on the kernel side"):
324	
325	      http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html
326	
327	   explained his reasoning:
328	
329	      http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html
330	      http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html
331	
332	   and, most importantly, designed and implemented the initramfs code.
333	
334	Future directions:
335	------------------
336	
337	Today (2.6.16), initramfs is always compiled in, but not always used.  The
338	kernel falls back to legacy boot code that is reached only if initramfs does
339	not contain an /init program.  The fallback is legacy code, there to ensure a
340	smooth transition and allowing early boot functionality to gradually move to
341	"early userspace" (I.E. initramfs).
342	
343	The move to early userspace is necessary because finding and mounting the real
344	root device is complex.  Root partitions can span multiple devices (raid or
345	separate journal).  They can be out on the network (requiring dhcp, setting a
346	specific MAC address, logging into a server, etc).  They can live on removable
347	media, with dynamically allocated major/minor numbers and persistent naming
348	issues requiring a full udev implementation to sort out.  They can be
349	compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
350	and so on.
351	
352	This kind of complexity (which inevitably includes policy) is rightly handled
353	in userspace.  Both klibc and busybox/uClibc are working on simple initramfs
354	packages to drop into a kernel build.
355	
356	The klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree.
357	The kernel's current early boot code (partition detection, etc) will probably
358	be migrated into a default initramfs, automatically created and used by the
359	kernel build.
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