1Using the initial RAM disk (initrd) 2=================================== 3 4Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and 5Hans Lermen <lermen@fgan.de> 6 7 8initrd provides the capability to load a RAM disk by the boot loader. 9This RAM disk can then be mounted as the root file system and programs 10can be run from it. Afterwards, a new root file system can be mounted 11from a different device. The previous root (from initrd) is then moved 12to a directory and can be subsequently unmounted. 13 14initrd is mainly designed to allow system startup to occur in two phases, 15where the kernel comes up with a minimum set of compiled-in drivers, and 16where additional modules are loaded from initrd. 17 18This document gives a brief overview of the use of initrd. A more detailed 19discussion of the boot process can be found in [#f1]_. 20 21 22Operation 23--------- 24 25When 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) if the root device is not ``/dev/ram0``, the old (deprecated) 31 change_root procedure is followed. see the "Obsolete root change 32 mechanism" section below. 33 4) root device is mounted. if it is ``/dev/ram0``, the initrd image is 34 then mounted as root 35 5) /sbin/init is executed (this can be any valid executable, including 36 shell scripts; it is run with uid 0 and can do basically everything 37 init can do). 38 6) init mounts the "real" root file system 39 7) init places the root file system at the root directory using the 40 pivot_root system call 41 8) init execs the ``/sbin/init`` on the new root filesystem, performing 42 the usual boot sequence 43 9) the initrd file system is removed 44 45Note that changing the root directory does not involve unmounting it. 46It is therefore possible to leave processes running on initrd during that 47procedure. Also note that file systems mounted under initrd continue to 48be accessible. 49 50 51Boot command-line options 52------------------------- 53 54initrd adds the following new options:: 55 56 initrd=<path> (e.g. LOADLIN) 57 58 Loads the specified file as the initial RAM disk. When using LILO, you 59 have to specify the RAM disk image file in /etc/lilo.conf, using the 60 INITRD configuration variable. 61 62 noinitrd 63 64 initrd data is preserved but it is not converted to a RAM disk and 65 the "normal" root file system is mounted. initrd data can be read 66 from /dev/initrd. Note that the data in initrd can have any structure 67 in this case and doesn't necessarily have to be a file system image. 68 This option is used mainly for debugging. 69 70 Note: /dev/initrd is read-only and it can only be used once. As soon 71 as the last process has closed it, all data is freed and /dev/initrd 72 can't be opened anymore. 73 74 root=/dev/ram0 75 76 initrd is mounted as root, and the normal boot procedure is followed, 77 with the RAM disk mounted as root. 78 79Compressed cpio images 80---------------------- 81 82Recent kernels have support for populating a ramdisk from a compressed cpio 83archive. On such systems, the creation of a ramdisk image doesn't need to 84involve special block devices or loopbacks; you merely create a directory on 85disk with the desired initrd content, cd to that directory, and run (as an 86example):: 87 88 find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img 89 90Examining the contents of an existing image file is just as simple:: 91 92 mkdir /tmp/imagefile 93 cd /tmp/imagefile 94 gzip -cd /boot/imagefile.img | cpio -imd --quiet 95 96Installation 97------------ 98 99First, a directory for the initrd file system has to be created on the 100"normal" root file system, e.g.:: 101 102 # mkdir /initrd 103 104The name is not relevant. More details can be found on the 105:manpage:`pivot_root(2)` man page. 106 107If the root file system is created during the boot procedure (i.e. if 108you're building an install floppy), the root file system creation 109procedure should create the ``/initrd`` directory. 110 111If initrd will not be mounted in some cases, its content is still 112accessible if the following device has been created:: 113 114 # mknod /dev/initrd b 1 250 115 # chmod 400 /dev/initrd 116 117Second, the kernel has to be compiled with RAM disk support and with 118support for the initial RAM disk enabled. Also, at least all components 119needed to execute programs from initrd (e.g. executable format and file 120system) must be compiled into the kernel. 121 122Third, you have to create the RAM disk image. This is done by creating a 123file system on a block device, copying files to it as needed, and then 124copying the content of the block device to the initrd file. With recent 125kernels, at least three types of devices are suitable for that: 126 127 - a floppy disk (works everywhere but it's painfully slow) 128 - a RAM disk (fast, but allocates physical memory) 129 - a loopback device (the most elegant solution) 130 131We'll describe the loopback device method: 132 133 1) make sure loopback block devices are configured into the kernel 134 2) create an empty file system of the appropriate size, e.g.:: 135 136 # dd if=/dev/zero of=initrd bs=300k count=1 137 # mke2fs -F -m0 initrd 138 139 (if space is critical, you may want to use the Minix FS instead of Ext2) 140 3) mount the file system, e.g.:: 141 142 # mount -t ext2 -o loop initrd /mnt 143 144 4) create the console device:: 145 146 # mkdir /mnt/dev 147 # mknod /mnt/dev/console c 5 1 148 149 5) copy all the files that are needed to properly use the initrd 150 environment. Don't forget the most important file, ``/sbin/init`` 151 152 .. note:: ``/sbin/init`` permissions must include "x" (execute). 153 154 6) correct operation the initrd environment can frequently be tested 155 even without rebooting with the command:: 156 157 # chroot /mnt /sbin/init 158 159 This is of course limited to initrds that do not interfere with the 160 general system state (e.g. by reconfiguring network interfaces, 161 overwriting mounted devices, trying to start already running demons, 162 etc. Note however that it is usually possible to use pivot_root in 163 such a chroot'ed initrd environment.) 164 7) unmount the file system:: 165 166 # umount /mnt 167 168 8) the initrd is now in the file "initrd". Optionally, it can now be 169 compressed:: 170 171 # gzip -9 initrd 172 173For experimenting with initrd, you may want to take a rescue floppy and 174only add a symbolic link from ``/sbin/init`` to ``/bin/sh``. Alternatively, you 175can try the experimental newlib environment [#f2]_ to create a small 176initrd. 177 178Finally, you have to boot the kernel and load initrd. Almost all Linux 179boot loaders support initrd. Since the boot process is still compatible 180with an older mechanism, the following boot command line parameters 181have to be given:: 182 183 root=/dev/ram0 rw 184 185(rw is only necessary if writing to the initrd file system.) 186 187With LOADLIN, you simply execute:: 188 189 LOADLIN <kernel> initrd=<disk_image> 190 191e.g.:: 192 193 LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw 194 195With LILO, you add the option ``INITRD=<path>`` to either the global section 196or to the section of the respective kernel in ``/etc/lilo.conf``, and pass 197the options using APPEND, e.g.:: 198 199 image = /bzImage 200 initrd = /boot/initrd.gz 201 append = "root=/dev/ram0 rw" 202 203and run ``/sbin/lilo`` 204 205For other boot loaders, please refer to the respective documentation. 206 207Now you can boot and enjoy using initrd. 208 209 210Changing the root device 211------------------------ 212 213When finished with its duties, init typically changes the root device 214and proceeds with starting the Linux system on the "real" root device. 215 216The procedure involves the following steps: 217 - mounting the new root file system 218 - turning it into the root file system 219 - removing all accesses to the old (initrd) root file system 220 - unmounting the initrd file system and de-allocating the RAM disk 221 222Mounting the new root file system is easy: it just needs to be mounted on 223a directory under the current root. Example:: 224 225 # mkdir /new-root 226 # mount -o ro /dev/hda1 /new-root 227 228The root change is accomplished with the pivot_root system call, which 229is also available via the ``pivot_root`` utility (see :manpage:`pivot_root(8)` 230man page; ``pivot_root`` is distributed with util-linux version 2.10h or higher 231[#f3]_). ``pivot_root`` moves the current root to a directory under the new 232root, and puts the new root at its place. The directory for the old root 233must exist before calling ``pivot_root``. Example:: 234 235 # cd /new-root 236 # mkdir initrd 237 # pivot_root . initrd 238 239Now, the init process may still access the old root via its 240executable, shared libraries, standard input/output/error, and its 241current root directory. All these references are dropped by the 242following command:: 243 244 # exec chroot . what-follows <dev/console >dev/console 2>&1 245 246Where what-follows is a program under the new root, e.g. ``/sbin/init`` 247If the new root file system will be used with udev and has no valid 248``/dev`` directory, udev must be initialized before invoking chroot in order 249to provide ``/dev/console``. 250 251Note: implementation details of pivot_root may change with time. In order 252to ensure compatibility, the following points should be observed: 253 254 - before calling pivot_root, the current directory of the invoking 255 process should point to the new root directory 256 - use . as the first argument, and the _relative_ path of the directory 257 for the old root as the second argument 258 - a chroot program must be available under the old and the new root 259 - chroot to the new root afterwards 260 - use relative paths for dev/console in the exec command 261 262Now, the initrd can be unmounted and the memory allocated by the RAM 263disk can be freed:: 264 265 # umount /initrd 266 # blockdev --flushbufs /dev/ram0 267 268It is also possible to use initrd with an NFS-mounted root, see the 269:manpage:`pivot_root(8)` man page for details. 270 271 272Usage scenarios 273--------------- 274 275The main motivation for implementing initrd was to allow for modular 276kernel configuration at system installation. The procedure would work 277as follows: 278 279 1) system boots from floppy or other media with a minimal kernel 280 (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and 281 loads initrd 282 2) ``/sbin/init`` determines what is needed to (1) mount the "real" root FS 283 (i.e. device type, device drivers, file system) and (2) the 284 distribution media (e.g. CD-ROM, network, tape, ...). This can be 285 done by asking the user, by auto-probing, or by using a hybrid 286 approach. 287 3) ``/sbin/init`` loads the necessary kernel modules 288 4) ``/sbin/init`` creates and populates the root file system (this doesn't 289 have to be a very usable system yet) 290 5) ``/sbin/init`` invokes ``pivot_root`` to change the root file system and 291 execs - via chroot - a program that continues the installation 292 6) the boot loader is installed 293 7) the boot loader is configured to load an initrd with the set of 294 modules that was used to bring up the system (e.g. ``/initrd`` can be 295 modified, then unmounted, and finally, the image is written from 296 ``/dev/ram0`` or ``/dev/rd/0`` to a file) 297 8) now the system is bootable and additional installation tasks can be 298 performed 299 300The key role of initrd here is to re-use the configuration data during 301normal system operation without requiring the use of a bloated "generic" 302kernel or re-compiling or re-linking the kernel. 303 304A second scenario is for installations where Linux runs on systems with 305different hardware configurations in a single administrative domain. In 306such cases, it is desirable to generate only a small set of kernels 307(ideally only one) and to keep the system-specific part of configuration 308information as small as possible. In this case, a common initrd could be 309generated with all the necessary modules. Then, only ``/sbin/init`` or a file 310read by it would have to be different. 311 312A third scenario is more convenient recovery disks, because information 313like the location of the root FS partition doesn't have to be provided at 314boot time, but the system loaded from initrd can invoke a user-friendly 315dialog and it can also perform some sanity checks (or even some form of 316auto-detection). 317 318Last not least, CD-ROM distributors may use it for better installation 319from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk 320via initrd from CD; or by booting via a loader like ``LOADLIN`` or directly 321from the CD-ROM, and loading the RAM disk from CD without need of 322floppies. 323 324 325Obsolete root change mechanism 326------------------------------ 327 328The following mechanism was used before the introduction of pivot_root. 329Current kernels still support it, but you should _not_ rely on its 330continued availability. 331 332It works by mounting the "real" root device (i.e. the one set with rdev 333in the kernel image or with root=... at the boot command line) as the 334root file system when linuxrc exits. The initrd file system is then 335unmounted, or, if it is still busy, moved to a directory ``/initrd``, if 336such a directory exists on the new root file system. 337 338In order to use this mechanism, you do not have to specify the boot 339command options root, init, or rw. (If specified, they will affect 340the real root file system, not the initrd environment.) 341 342If /proc is mounted, the "real" root device can be changed from within 343linuxrc by writing the number of the new root FS device to the special 344file /proc/sys/kernel/real-root-dev, e.g.:: 345 346 # echo 0x301 >/proc/sys/kernel/real-root-dev 347 348Note that the mechanism is incompatible with NFS and similar file 349systems. 350 351This old, deprecated mechanism is commonly called ``change_root``, while 352the new, supported mechanism is called ``pivot_root``. 353 354 355Mixed change_root and pivot_root mechanism 356------------------------------------------ 357 358In case you did not want to use ``root=/dev/ram0`` to trigger the pivot_root 359mechanism, you may create both ``/linuxrc`` and ``/sbin/init`` in your initrd 360image. 361 362``/linuxrc`` would contain only the following:: 363 364 #! /bin/sh 365 mount -n -t proc proc /proc 366 echo 0x0100 >/proc/sys/kernel/real-root-dev 367 umount -n /proc 368 369Once linuxrc exited, the kernel would mount again your initrd as root, 370this time executing ``/sbin/init``. Again, it would be the duty of this init 371to build the right environment (maybe using the ``root= device`` passed on 372the cmdline) before the final execution of the real ``/sbin/init``. 373 374 375Resources 376--------- 377 378.. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future" 379 http://www.almesberger.net/cv/papers/ols2k-9.ps.gz 380.. [#f2] newlib package (experimental), with initrd example 381 https://www.sourceware.org/newlib/ 382.. [#f3] util-linux: Miscellaneous utilities for Linux 383 https://www.kernel.org/pub/linux/utils/util-linux/ 384
