1Using the initial RAM disk (initrd) 2=================================== 3 4Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and 5 Hans 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 [1]. 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 88find . | 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 92mkdir /tmp/imagefile 93cd /tmp/imagefile 94gzip -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 pivot_root(2) 105man 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 # dd if=/dev/zero of=initrd bs=300k count=1 136 # mke2fs -F -m0 initrd 137 (if space is critical, you may want to use the Minix FS instead of Ext2) 138 3) mount the file system, e.g. 139 # mount -t ext2 -o loop initrd /mnt 140 4) create the console device: 141 # mkdir /mnt/dev 142 # mknod /mnt/dev/console c 5 1 143 5) copy all the files that are needed to properly use the initrd 144 environment. Don't forget the most important file, /sbin/init 145 Note that /sbin/init's permissions must include "x" (execute). 146 6) correct operation the initrd environment can frequently be tested 147 even without rebooting with the command 148 # chroot /mnt /sbin/init 149 This is of course limited to initrds that do not interfere with the 150 general system state (e.g. by reconfiguring network interfaces, 151 overwriting mounted devices, trying to start already running demons, 152 etc. Note however that it is usually possible to use pivot_root in 153 such a chroot'ed initrd environment.) 154 7) unmount the file system 155 # umount /mnt 156 8) the initrd is now in the file "initrd". Optionally, it can now be 157 compressed 158 # gzip -9 initrd 159 160For experimenting with initrd, you may want to take a rescue floppy and 161only add a symbolic link from /sbin/init to /bin/sh. Alternatively, you 162can try the experimental newlib environment [2] to create a small 163initrd. 164 165Finally, you have to boot the kernel and load initrd. Almost all Linux 166boot loaders support initrd. Since the boot process is still compatible 167with an older mechanism, the following boot command line parameters 168have to be given: 169 170 root=/dev/ram0 rw 171 172(rw is only necessary if writing to the initrd file system.) 173 174With LOADLIN, you simply execute 175 176 LOADLIN <kernel> initrd=<disk_image> 177e.g. LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw 178 179With LILO, you add the option INITRD=<path> to either the global section 180or to the section of the respective kernel in /etc/lilo.conf, and pass 181the options using APPEND, e.g. 182 183 image = /bzImage 184 initrd = /boot/initrd.gz 185 append = "root=/dev/ram0 rw" 186 187and run /sbin/lilo 188 189For other boot loaders, please refer to the respective documentation. 190 191Now you can boot and enjoy using initrd. 192 193 194Changing the root device 195------------------------ 196 197When finished with its duties, init typically changes the root device 198and proceeds with starting the Linux system on the "real" root device. 199 200The procedure involves the following steps: 201 - mounting the new root file system 202 - turning it into the root file system 203 - removing all accesses to the old (initrd) root file system 204 - unmounting the initrd file system and de-allocating the RAM disk 205 206Mounting the new root file system is easy: it just needs to be mounted on 207a directory under the current root. Example: 208 209# mkdir /new-root 210# mount -o ro /dev/hda1 /new-root 211 212The root change is accomplished with the pivot_root system call, which 213is also available via the pivot_root utility (see pivot_root(8) man 214page; pivot_root is distributed with util-linux version 2.10h or higher 215[3]). pivot_root moves the current root to a directory under the new 216root, and puts the new root at its place. The directory for the old root 217must exist before calling pivot_root. Example: 218 219# cd /new-root 220# mkdir initrd 221# pivot_root . initrd 222 223Now, the init process may still access the old root via its 224executable, shared libraries, standard input/output/error, and its 225current root directory. All these references are dropped by the 226following command: 227 228# exec chroot . what-follows <dev/console >dev/console 2>&1 229 230Where what-follows is a program under the new root, e.g. /sbin/init 231If the new root file system will be used with udev and has no valid 232/dev directory, udev must be initialized before invoking chroot in order 233to provide /dev/console. 234 235Note: implementation details of pivot_root may change with time. In order 236to ensure compatibility, the following points should be observed: 237 238 - before calling pivot_root, the current directory of the invoking 239 process should point to the new root directory 240 - use . as the first argument, and the _relative_ path of the directory 241 for the old root as the second argument 242 - a chroot program must be available under the old and the new root 243 - chroot to the new root afterwards 244 - use relative paths for dev/console in the exec command 245 246Now, the initrd can be unmounted and the memory allocated by the RAM 247disk can be freed: 248 249# umount /initrd 250# blockdev --flushbufs /dev/ram0 251 252It is also possible to use initrd with an NFS-mounted root, see the 253pivot_root(8) man page for details. 254 255 256Usage scenarios 257--------------- 258 259The main motivation for implementing initrd was to allow for modular 260kernel configuration at system installation. The procedure would work 261as follows: 262 263 1) system boots from floppy or other media with a minimal kernel 264 (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and 265 loads initrd 266 2) /sbin/init determines what is needed to (1) mount the "real" root FS 267 (i.e. device type, device drivers, file system) and (2) the 268 distribution media (e.g. CD-ROM, network, tape, ...). This can be 269 done by asking the user, by auto-probing, or by using a hybrid 270 approach. 271 3) /sbin/init loads the necessary kernel modules 272 4) /sbin/init creates and populates the root file system (this doesn't 273 have to be a very usable system yet) 274 5) /sbin/init invokes pivot_root to change the root file system and 275 execs - via chroot - a program that continues the installation 276 6) the boot loader is installed 277 7) the boot loader is configured to load an initrd with the set of 278 modules that was used to bring up the system (e.g. /initrd can be 279 modified, then unmounted, and finally, the image is written from 280 /dev/ram0 or /dev/rd/0 to a file) 281 8) now the system is bootable and additional installation tasks can be 282 performed 283 284The key role of initrd here is to re-use the configuration data during 285normal system operation without requiring the use of a bloated "generic" 286kernel or re-compiling or re-linking the kernel. 287 288A second scenario is for installations where Linux runs on systems with 289different hardware configurations in a single administrative domain. In 290such cases, it is desirable to generate only a small set of kernels 291(ideally only one) and to keep the system-specific part of configuration 292information as small as possible. In this case, a common initrd could be 293generated with all the necessary modules. Then, only /sbin/init or a file 294read by it would have to be different. 295 296A third scenario is more convenient recovery disks, because information 297like the location of the root FS partition doesn't have to be provided at 298boot time, but the system loaded from initrd can invoke a user-friendly 299dialog and it can also perform some sanity checks (or even some form of 300auto-detection). 301 302Last not least, CD-ROM distributors may use it for better installation 303from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk 304via initrd from CD; or by booting via a loader like LOADLIN or directly 305from the CD-ROM, and loading the RAM disk from CD without need of 306floppies. 307 308 309Obsolete root change mechanism 310------------------------------ 311 312The following mechanism was used before the introduction of pivot_root. 313Current kernels still support it, but you should _not_ rely on its 314continued availability. 315 316It works by mounting the "real" root device (i.e. the one set with rdev 317in the kernel image or with root=... at the boot command line) as the 318root file system when linuxrc exits. The initrd file system is then 319unmounted, or, if it is still busy, moved to a directory /initrd, if 320such a directory exists on the new root file system. 321 322In order to use this mechanism, you do not have to specify the boot 323command options root, init, or rw. (If specified, they will affect 324the real root file system, not the initrd environment.) 325 326If /proc is mounted, the "real" root device can be changed from within 327linuxrc by writing the number of the new root FS device to the special 328file /proc/sys/kernel/real-root-dev, e.g. 329 330 # echo 0x301 >/proc/sys/kernel/real-root-dev 331 332Note that the mechanism is incompatible with NFS and similar file 333systems. 334 335This old, deprecated mechanism is commonly called "change_root", while 336the new, supported mechanism is called "pivot_root". 337 338 339Mixed change_root and pivot_root mechanism 340------------------------------------------ 341 342In case you did not want to use root=/dev/ram0 to trigger the pivot_root 343mechanism, you may create both /linuxrc and /sbin/init in your initrd image. 344 345/linuxrc would contain only the following: 346 347#! /bin/sh 348mount -n -t proc proc /proc 349echo 0x0100 >/proc/sys/kernel/real-root-dev 350umount -n /proc 351 352Once linuxrc exited, the kernel would mount again your initrd as root, 353this time executing /sbin/init. Again, it would be the duty of this init 354to build the right environment (maybe using the root= device passed on 355the cmdline) before the final execution of the real /sbin/init. 356 357 358Resources 359--------- 360 361[1] Almesberger, Werner; "Booting Linux: The History and the Future" 362 http://www.almesberger.net/cv/papers/ols2k-9.ps.gz 363[2] newlib package (experimental), with initrd example 364 http://sources.redhat.com/newlib/ 365[3] util-linux: Miscellaneous utilities for Linux 366 http://www.kernel.org/pub/linux/utils/util-linux/ 367