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Tag kernel26-2.6.12-alx-r9
1 | Using the initial RAM disk (initrd) |
2 | =================================== |
3 | |
4 | Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and |
5 | Hans Lermen <lermen@fgan.de> |
6 | |
7 | |
8 | initrd provides the capability to load a RAM disk by the boot loader. |
9 | This RAM disk can then be mounted as the root file system and programs |
10 | can be run from it. Afterwards, a new root file system can be mounted |
11 | from a different device. The previous root (from initrd) is then moved |
12 | to a directory and can be subsequently unmounted. |
13 | |
14 | initrd is mainly designed to allow system startup to occur in two phases, |
15 | where the kernel comes up with a minimum set of compiled-in drivers, and |
16 | where additional modules are loaded from initrd. |
17 | |
18 | This document gives a brief overview of the use of initrd. A more detailed |
19 | discussion of the boot process can be found in [1]. |
20 | |
21 | |
22 | Operation |
23 | --------- |
24 | |
25 | When using initrd, the system typically boots as follows: |
26 | |
27 | 1) the boot loader loads the kernel and the initial RAM disk |
28 | 2) the kernel converts initrd into a "normal" RAM disk and |
29 | frees the memory used by initrd |
30 | 3) initrd is mounted read-write as root |
31 | 4) /linuxrc is executed (this can be any valid executable, including |
32 | shell scripts; it is run with uid 0 and can do basically everything |
33 | init can do) |
34 | 5) linuxrc mounts the "real" root file system |
35 | 6) linuxrc places the root file system at the root directory using the |
36 | pivot_root system call |
37 | 7) the usual boot sequence (e.g. invocation of /sbin/init) is performed |
38 | on the root file system |
39 | 8) the initrd file system is removed |
40 | |
41 | Note that changing the root directory does not involve unmounting it. |
42 | It is therefore possible to leave processes running on initrd during that |
43 | procedure. Also note that file systems mounted under initrd continue to |
44 | be accessible. |
45 | |
46 | |
47 | Boot command-line options |
48 | ------------------------- |
49 | |
50 | initrd adds the following new options: |
51 | |
52 | initrd=<path> (e.g. LOADLIN) |
53 | |
54 | Loads the specified file as the initial RAM disk. When using LILO, you |
55 | have to specify the RAM disk image file in /etc/lilo.conf, using the |
56 | INITRD configuration variable. |
57 | |
58 | noinitrd |
59 | |
60 | initrd data is preserved but it is not converted to a RAM disk and |
61 | the "normal" root file system is mounted. initrd data can be read |
62 | from /dev/initrd. Note that the data in initrd can have any structure |
63 | in this case and doesn't necessarily have to be a file system image. |
64 | This option is used mainly for debugging. |
65 | |
66 | Note: /dev/initrd is read-only and it can only be used once. As soon |
67 | as the last process has closed it, all data is freed and /dev/initrd |
68 | can't be opened anymore. |
69 | |
70 | root=/dev/ram0 (without devfs) |
71 | root=/dev/rd/0 (with devfs) |
72 | |
73 | initrd is mounted as root, and the normal boot procedure is followed, |
74 | with the RAM disk still mounted as root. |
75 | |
76 | |
77 | Installation |
78 | ------------ |
79 | |
80 | First, a directory for the initrd file system has to be created on the |
81 | "normal" root file system, e.g. |
82 | |
83 | # mkdir /initrd |
84 | |
85 | The name is not relevant. More details can be found on the pivot_root(2) |
86 | man page. |
87 | |
88 | If the root file system is created during the boot procedure (i.e. if |
89 | you're building an install floppy), the root file system creation |
90 | procedure should create the /initrd directory. |
91 | |
92 | If initrd will not be mounted in some cases, its content is still |
93 | accessible if the following device has been created (note that this |
94 | does not work if using devfs): |
95 | |
96 | # mknod /dev/initrd b 1 250 |
97 | # chmod 400 /dev/initrd |
98 | |
99 | Second, the kernel has to be compiled with RAM disk support and with |
100 | support for the initial RAM disk enabled. Also, at least all components |
101 | needed to execute programs from initrd (e.g. executable format and file |
102 | system) must be compiled into the kernel. |
103 | |
104 | Third, you have to create the RAM disk image. This is done by creating a |
105 | file system on a block device, copying files to it as needed, and then |
106 | copying the content of the block device to the initrd file. With recent |
107 | kernels, at least three types of devices are suitable for that: |
108 | |
109 | - a floppy disk (works everywhere but it's painfully slow) |
110 | - a RAM disk (fast, but allocates physical memory) |
111 | - a loopback device (the most elegant solution) |
112 | |
113 | We'll describe the loopback device method: |
114 | |
115 | 1) make sure loopback block devices are configured into the kernel |
116 | 2) create an empty file system of the appropriate size, e.g. |
117 | # dd if=/dev/zero of=initrd bs=300k count=1 |
118 | # mke2fs -F -m0 initrd |
119 | (if space is critical, you may want to use the Minix FS instead of Ext2) |
120 | 3) mount the file system, e.g. |
121 | # mount -t ext2 -o loop initrd /mnt |
122 | 4) create the console device (not necessary if using devfs, but it can't |
123 | hurt to do it anyway): |
124 | # mkdir /mnt/dev |
125 | # mknod /mnt/dev/console c 5 1 |
126 | 5) copy all the files that are needed to properly use the initrd |
127 | environment. Don't forget the most important file, /linuxrc |
128 | Note that /linuxrc's permissions must include "x" (execute). |
129 | 6) correct operation the initrd environment can frequently be tested |
130 | even without rebooting with the command |
131 | # chroot /mnt /linuxrc |
132 | This is of course limited to initrds that do not interfere with the |
133 | general system state (e.g. by reconfiguring network interfaces, |
134 | overwriting mounted devices, trying to start already running demons, |
135 | etc. Note however that it is usually possible to use pivot_root in |
136 | such a chroot'ed initrd environment.) |
137 | 7) unmount the file system |
138 | # umount /mnt |
139 | 8) the initrd is now in the file "initrd". Optionally, it can now be |
140 | compressed |
141 | # gzip -9 initrd |
142 | |
143 | For experimenting with initrd, you may want to take a rescue floppy and |
144 | only add a symbolic link from /linuxrc to /bin/sh. Alternatively, you |
145 | can try the experimental newlib environment [2] to create a small |
146 | initrd. |
147 | |
148 | Finally, you have to boot the kernel and load initrd. Almost all Linux |
149 | boot loaders support initrd. Since the boot process is still compatible |
150 | with an older mechanism, the following boot command line parameters |
151 | have to be given: |
152 | |
153 | root=/dev/ram0 init=/linuxrc rw |
154 | |
155 | if not using devfs, or |
156 | |
157 | root=/dev/rd/0 init=/linuxrc rw |
158 | |
159 | if using devfs. (rw is only necessary if writing to the initrd file |
160 | system.) |
161 | |
162 | With LOADLIN, you simply execute |
163 | |
164 | LOADLIN <kernel> initrd=<disk_image> |
165 | e.g. LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 |
166 | init=/linuxrc rw |
167 | |
168 | With LILO, you add the option INITRD=<path> to either the global section |
169 | or to the section of the respective kernel in /etc/lilo.conf, and pass |
170 | the options using APPEND, e.g. |
171 | |
172 | image = /bzImage |
173 | initrd = /boot/initrd.gz |
174 | append = "root=/dev/ram0 init=/linuxrc rw" |
175 | |
176 | and run /sbin/lilo |
177 | |
178 | For other boot loaders, please refer to the respective documentation. |
179 | |
180 | Now you can boot and enjoy using initrd. |
181 | |
182 | |
183 | Changing the root device |
184 | ------------------------ |
185 | |
186 | When finished with its duties, linuxrc typically changes the root device |
187 | and proceeds with starting the Linux system on the "real" root device. |
188 | |
189 | The procedure involves the following steps: |
190 | - mounting the new root file system |
191 | - turning it into the root file system |
192 | - removing all accesses to the old (initrd) root file system |
193 | - unmounting the initrd file system and de-allocating the RAM disk |
194 | |
195 | Mounting the new root file system is easy: it just needs to be mounted on |
196 | a directory under the current root. Example: |
197 | |
198 | # mkdir /new-root |
199 | # mount -o ro /dev/hda1 /new-root |
200 | |
201 | The root change is accomplished with the pivot_root system call, which |
202 | is also available via the pivot_root utility (see pivot_root(8) man |
203 | page; pivot_root is distributed with util-linux version 2.10h or higher |
204 | [3]). pivot_root moves the current root to a directory under the new |
205 | root, and puts the new root at its place. The directory for the old root |
206 | must exist before calling pivot_root. Example: |
207 | |
208 | # cd /new-root |
209 | # mkdir initrd |
210 | # pivot_root . initrd |
211 | |
212 | Now, the linuxrc process may still access the old root via its |
213 | executable, shared libraries, standard input/output/error, and its |
214 | current root directory. All these references are dropped by the |
215 | following command: |
216 | |
217 | # exec chroot . what-follows <dev/console >dev/console 2>&1 |
218 | |
219 | Where what-follows is a program under the new root, e.g. /sbin/init |
220 | If the new root file system will be used with devfs and has no valid |
221 | /dev directory, devfs must be mounted before invoking chroot in order to |
222 | provide /dev/console. |
223 | |
224 | Note: implementation details of pivot_root may change with time. In order |
225 | to ensure compatibility, the following points should be observed: |
226 | |
227 | - before calling pivot_root, the current directory of the invoking |
228 | process should point to the new root directory |
229 | - use . as the first argument, and the _relative_ path of the directory |
230 | for the old root as the second argument |
231 | - a chroot program must be available under the old and the new root |
232 | - chroot to the new root afterwards |
233 | - use relative paths for dev/console in the exec command |
234 | |
235 | Now, the initrd can be unmounted and the memory allocated by the RAM |
236 | disk can be freed: |
237 | |
238 | # umount /initrd |
239 | # blockdev --flushbufs /dev/ram0 # /dev/rd/0 if using devfs |
240 | |
241 | It is also possible to use initrd with an NFS-mounted root, see the |
242 | pivot_root(8) man page for details. |
243 | |
244 | Note: if linuxrc or any program exec'ed from it terminates for some |
245 | reason, the old change_root mechanism is invoked (see section "Obsolete |
246 | root change mechanism"). |
247 | |
248 | |
249 | Usage scenarios |
250 | --------------- |
251 | |
252 | The main motivation for implementing initrd was to allow for modular |
253 | kernel configuration at system installation. The procedure would work |
254 | as follows: |
255 | |
256 | 1) system boots from floppy or other media with a minimal kernel |
257 | (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and |
258 | loads initrd |
259 | 2) /linuxrc determines what is needed to (1) mount the "real" root FS |
260 | (i.e. device type, device drivers, file system) and (2) the |
261 | distribution media (e.g. CD-ROM, network, tape, ...). This can be |
262 | done by asking the user, by auto-probing, or by using a hybrid |
263 | approach. |
264 | 3) /linuxrc loads the necessary kernel modules |
265 | 4) /linuxrc creates and populates the root file system (this doesn't |
266 | have to be a very usable system yet) |
267 | 5) /linuxrc invokes pivot_root to change the root file system and |
268 | execs - via chroot - a program that continues the installation |
269 | 6) the boot loader is installed |
270 | 7) the boot loader is configured to load an initrd with the set of |
271 | modules that was used to bring up the system (e.g. /initrd can be |
272 | modified, then unmounted, and finally, the image is written from |
273 | /dev/ram0 or /dev/rd/0 to a file) |
274 | 8) now the system is bootable and additional installation tasks can be |
275 | performed |
276 | |
277 | The key role of initrd here is to re-use the configuration data during |
278 | normal system operation without requiring the use of a bloated "generic" |
279 | kernel or re-compiling or re-linking the kernel. |
280 | |
281 | A second scenario is for installations where Linux runs on systems with |
282 | different hardware configurations in a single administrative domain. In |
283 | such cases, it is desirable to generate only a small set of kernels |
284 | (ideally only one) and to keep the system-specific part of configuration |
285 | information as small as possible. In this case, a common initrd could be |
286 | generated with all the necessary modules. Then, only /linuxrc or a file |
287 | read by it would have to be different. |
288 | |
289 | A third scenario are more convenient recovery disks, because information |
290 | like the location of the root FS partition doesn't have to be provided at |
291 | boot time, but the system loaded from initrd can invoke a user-friendly |
292 | dialog and it can also perform some sanity checks (or even some form of |
293 | auto-detection). |
294 | |
295 | Last not least, CD-ROM distributors may use it for better installation |
296 | from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk |
297 | via initrd from CD; or by booting via a loader like LOADLIN or directly |
298 | from the CD-ROM, and loading the RAM disk from CD without need of |
299 | floppies. |
300 | |
301 | |
302 | Obsolete root change mechanism |
303 | ------------------------------ |
304 | |
305 | The following mechanism was used before the introduction of pivot_root. |
306 | Current kernels still support it, but you should _not_ rely on its |
307 | continued availability. |
308 | |
309 | It works by mounting the "real" root device (i.e. the one set with rdev |
310 | in the kernel image or with root=... at the boot command line) as the |
311 | root file system when linuxrc exits. The initrd file system is then |
312 | unmounted, or, if it is still busy, moved to a directory /initrd, if |
313 | such a directory exists on the new root file system. |
314 | |
315 | In order to use this mechanism, you do not have to specify the boot |
316 | command options root, init, or rw. (If specified, they will affect |
317 | the real root file system, not the initrd environment.) |
318 | |
319 | If /proc is mounted, the "real" root device can be changed from within |
320 | linuxrc by writing the number of the new root FS device to the special |
321 | file /proc/sys/kernel/real-root-dev, e.g. |
322 | |
323 | # echo 0x301 >/proc/sys/kernel/real-root-dev |
324 | |
325 | Note that the mechanism is incompatible with NFS and similar file |
326 | systems. |
327 | |
328 | This old, deprecated mechanism is commonly called "change_root", while |
329 | the new, supported mechanism is called "pivot_root". |
330 | |
331 | |
332 | Resources |
333 | --------- |
334 | |
335 | [1] Almesberger, Werner; "Booting Linux: The History and the Future" |
336 | http://www.almesberger.net/cv/papers/ols2k-9.ps.gz |
337 | [2] newlib package (experimental), with initrd example |
338 | http://sources.redhat.com/newlib/ |
339 | [3] Brouwer, Andries; "util-linux: Miscellaneous utilities for Linux" |
340 | ftp://ftp.win.tue.nl/pub/linux-local/utils/util-linux/ |