| Rev | Author | # | Line |
|---|---|---|---|
| 1 | perry | 1 | !!NAME |
| 4 | CraigBox | 2 | initrd - boot loader initialized RAM disk |
| 1 | perry | 3 | |
| 4 | !!DESCRIPTION | ||
| 4 | CraigBox | 5 | The special file __/dev/initrd __ is a read-only block device. Device __/dev/initrd__ is a RAM disk that is initialized (e.g. loaded) by the boot loader before the [Kernel] is started. The kernel then can use the the block device __/dev/initrd__ 's contents for a two phased system boot-up. |
| 1 | perry | 6 | |
| 4 | CraigBox | 7 | In the first boot-up phase, the kernel starts up and mounts an initial root FileSystem from the contents of __/dev/initrd __ (e.g. RAM disk initialized by the BootLoader). In the second phase, additional drivers or other modules are loaded from the initial root device's contents. After loading the additional modules, a new root file system (i.e. the normal root file system) is [Mount]ed from a different device. |
| 1 | perry | 8 | |
| 9 | !!BOOT-UP OPERATION | ||
| 4 | CraigBox | 10 | When booting up with __initrd__ , the system boots as follows: |
| 1 | perry | 11 | |
| 4 | CraigBox | 12 | # The boot loader loads the kernel program and __/dev/initrd__ 's contents into memory. |
| 13 | # On kernel startup, the kernel uncompresses and copies the contents of the device __/dev/initrd__ onto device __/dev/ram0 __ and then frees the memory used by __/dev/initrd__ . | ||
| 14 | # The kernel then read-write mounts device __/dev/ram0 __ as the initial root file system. | ||
| 15 | # If the indicated normal root file system is also the initial root file-system (e.g. __/dev/ram0__ ) then the kernel skips to the last step for the usual boot sequence. | ||
| 16 | # If the executable file __/linuxrc__ is present in the initial root file-system, __/linuxrc__ is executed with uid 0. (The file __/linuxrc__ must have executable permission. The file __/linuxrc__ can be any valid executable, including a shell script.) | ||
| 17 | # If __/linuxrc__ is not executed or when __/linuxrc__ terminates, the normal root file system is mounted. (If __/linuxrc__ exits with any file-systems mounted on the initial root file-system, then the behavior of the kernel is __UNSPECIFIED__ . See the __NOTES__ section for the current kernel behavior.) | ||
| 18 | # If the normal root file has directory __/initrd__ , device __/dev/ram0__ is moved from __/__ to __/initrd__ . Otherwise if directory __/initrd__ does not exist device __/dev/ram0__ is unmounted. (When moved from __/__ to __/initrd__ , __/dev/ram0__ is not unmounted and therefore processes can remain running from __/dev/ram0__ . If directory __/initrd__ does not exist on the normal root file-system and any processes remain running from __/dev/ram0__ when __/linuxrc__ exits, the behavior of the kernel is __UNSPECIFIED__ . See the __NOTES__ section for the current kernel behavior.) | ||
| 19 | # The usual boot sequence (e.g. invocation of __/sbin/init__ ) is performed on the normal root file system. | ||
| 1 | perry | 20 | |
| 21 | !!OPTIONS | ||
| 4 | CraigBox | 22 | The following boot loader options when used with __initrd__ , affect the kernel's boot-up operation: |
| 23 | ;__initrd=__ ''filename'' : Specifies the file to load as the contents of __/dev/initrd__ . For __LOADLIN__ this is a command line option. For __LILO__ __you__ have __to__ use __this__ command __in__ the __LILO__ configuration file __/etc/lilo.config__ . The filename specified with this option will typically be a gzipped file-system image. | ||
| 24 | ;__noinitrd__ : This boot time option disables the two phase boot-up operation. The kernel performs the usual boot sequence as if __/dev/initrd __ was not initialized. With this option, any contents of __/dev/initrd __ loaded into memory by the boot loader contents are preserved. This option permits the contents of __/dev/initrd__ to be any data and need not be limited to a file system image. However, device __/dev/initrd __ is read-only and can be read only one time after system startup. | ||
| 25 | ;__root=__ ''device-name'' : Specifies the device to be used as the normal root file system. For __LOADLIN__ this is a command line option. For __LILO__ __this__ is __a__ boot __time__ option __or__ can be used as an option line in the __LILO__ configuration file __/etc/lilo.config__ . The device specified by the this option must be a mountable device having a suitable root file-system. | ||
| 1 | perry | 26 | |
| 27 | !!CHANGING THE NORMAL ROOT FILE SYSTEM | ||
| 4 | CraigBox | 28 | By default, the kernel's settings (e.g. set in the kernel file with __rdev__ or compiled into the kernel file), or the boot loader option setting is used for the normal root file systems. For a NFS-mounted normal root file system, one has to use the __nfs_root_name__ and __nfs_root_addrs__ boot options to give the NFS settings. For more information on NFS-mounted root see the kernel documentation file __nfsroot.txt__ . For more information on setting the root file system also see the __LILO__ and __LOADLIN__ documentation. |
| 1 | perry | 29 | |
| 4 | CraigBox | 30 | It is also possible for the __/linuxrc__ executable to change the normal root device. For __/linuxrc__ to change the normal root device, __/proc__ must be mounted. After mounting __/proc__ , __/linuxrc__ changes the normal root device by writing into the proc files __/proc/sys/kernel/real-root-dev__ , __/proc/sys/kernel/nfs-root-name__ , and __/proc/sys/kernel/nfs-root-addrs__ . For a physical root device, the root device is changed by having __/linuxrc__ write the new root file system device number into __/proc/sys/kernel/real-root-dev__ . For a NFS root file system, the root device is changed by having __/linuxrc__ write the NFS setting into files __/proc/sys/kernel/nfs-root-name__ and __/proc/sys/kernel/nfs-root-addrs__ and then writing 0xff (e.g. the pseudo-NFS-device number) into file __/proc/sys/kernel/real-root-dev__ . For example, the following shell command line would change the normal root device to __/dev/hdb1__ : |
| 1 | perry | 31 | |
| 4 | CraigBox | 32 | echo 0x365 >/proc/sys/kernel/real-root-dev |
| 1 | perry | 33 | |
| 4 | CraigBox | 34 | For a NFS example, the following shell command lines would change the normal root device to the NFS directory __/var/nfsroot__ on a local networked NFS server with IP number 193.8.232.7 for a system with IP number 193.8.232.7 and named 'idefix': |
| 35 | echo /var/nfsroot >/proc/sys/kernel/nfs-root-name | ||
| 36 | echo 193.8.232.2:193.8.232.7::255.255.255.0:idefix \ | ||
| 37 | >/proc/sys/kernel/nfs-root-addrs | ||
| 38 | echo 255 >/proc/sys/kernel/real-root-dev | ||
| 1 | perry | 39 | |
| 40 | |||
| 41 | |||
| 42 | !!USAGE | ||
| 4 | CraigBox | 43 | The main motivation for implementing __initrd__ was to allow for modular kernel configuration at system installation. |
| 1 | perry | 44 | |
| 4 | CraigBox | 45 | A possible system installation scenario is as follows: |
| 1 | perry | 46 | |
| 4 | CraigBox | 47 | # The loader program boots from floppy or other media with a minimal kernel (e.g. support for __/dev/ram__ , __/dev/initrd__ , and the ext2 file-system) and loads __/dev/initrd__ __with__ a __gzipped__ version __of__ the __initial__ file-system. |
| 48 | # The executable __/linuxrc__ determines what is needed to (1) mount the normal root file-system (i.e. device type, device drivers, file system) and (2) the distribution media (e.g. CD-ROM, network, tape, ...). This can be done by asking the user, by auto-probing, or by using a hybrid approach. | ||
| 49 | # The executable __/linuxrc__ loads the necessary modules from the initial root file-system. | ||
| 50 | # The executable __/linuxrc__ creates and populates the root file system. (At this stage the normal root file system does not have to be a completed system yet.) | ||
| 51 | # The executable __/linuxrc__ sets __/proc/sys/kernel/real-root-dev,__ unmount __/proc__ , the normal root file system and any other file systems it has mounted, and then terminates. | ||
| 52 | # The kernel then mounts the normal root file system. | ||
| 53 | # Now that the file system is accessible and intact, the boot loader can be installed. | ||
| 54 | # The boot loader is configured to load into __/dev/initrd__ a file system with the set of modules that was used to bring up the system. (e.g. Device __/dev/ram0__ can be modified, then unmounted, and finally, the image is written from __/dev/ram0__ to a file.) | ||
| 55 | # The system is now bootable and additional installation tasks can be performed. | ||
| 1 | perry | 56 | |
| 57 | |||
| 4 | CraigBox | 58 | The key role of __/dev/initrd__ in the above is to re-use the configuration data during normal system operation without requiring initial kernel selection, a large generic kernel or, recompiling the kernel. |
| 1 | perry | 59 | |
| 4 | CraigBox | 60 | A second scenario is for installations where Linux runs on systems with different hardware configurations in a single administrative network. In such cases, it may be desirable to use only a small set of kernels (ideally only one) and to keep the system-specific part of configuration information as small as possible. In this case, create a common file with all needed modules. Then, only the the __/linuxrc__ file or a file executed by __/linuxrc__ would be different. |
| 1 | perry | 61 | |
| 4 | CraigBox | 62 | A third scenario is more convenient recovery disks. Because information like the location of the root file-system partition is not needed at boot time, the system loaded from __/dev/initrd __ can use a dialog and/or auto-detection followed by a possible sanity check. |
| 1 | perry | 63 | |
| 4 | CraigBox | 64 | Last but not least, Linux distributions on CD-ROM may use __initrd__ for easy installation from the CD-ROM. The distribution can use __LOADLIN__ to directly load __/dev/initrd__ from CD-ROM without the need of any floppies. The distribution could also use a __LILO__ boot floppy and then bootstrap a bigger ram disk via __/dev/initrd__ from the CD-ROM. |
| 1 | perry | 65 | |
| 66 | !!CONFIGURATION | ||
| 4 | CraigBox | 67 | The __/dev/initrd __ is a read-only block device assigned major number 1 and minor number 250. Typically __/dev/initrd__ is owned by __root.disk __ with mode 0400 (read access by root only). If the Linux system does not have __/dev/initrd__ already created, it can be created with the following commands: |
| 68 | __ | ||
| 1 | perry | 69 | |
| 4 | CraigBox | 70 | mknod -m 400 /dev/initrd b 1 250 |
| 1 | perry | 71 | |
| 4 | CraigBox | 72 | chown root:disk /dev/initrd |
| 1 | perry | 73 | |
| 4 | CraigBox | 74 | Also, support for both "RAM disk" and "Initial RAM disk" (e.g. __CONFIG_BLK_DEV_RAM=y__ and __CONFIG_BLK_DEV_INITRD=y__ ) support must be compiled directly into the Linux kernel to use __/dev/initrd__ . When using __/dev/initrd__ , the RAM disk driver cannot be loaded as a module. |
| 1 | perry | 75 | |
| 76 | !!FILES | ||
| 4 | CraigBox | 77 | ''/dev/initrd'' |
| 78 | ''/dev/ram0'' | ||
| 79 | ''/linuxrc'' | ||
| 80 | ''/initrd'' | ||
| 1 | perry | 81 | |
| 82 | !!SEE ALSO | ||
| 4 | CraigBox | 83 | chown(1), mknod(1), __/dev/ram__ (4), freeramdisk(8), rdev(8), The documentation file ''initrd.txt'' in the kernel source package, the LILO documentation, the LOADLIN documentation, the SYSLINUX documentation. |
| 1 | perry | 84 | |
| 85 | !!NOTES | ||
| 4 | CraigBox | 86 | 1. With the current kernel, any file systems that remain mounted when __/dev/ram0__ is moved from __/__ to __/initrd__ continue to be accessible. However, the __/proc/mounts__ entries are not updated. |
| 1 | perry | 87 | |
| 4 | CraigBox | 88 | 2. With the current kernel, if directory __/initrd__ does not exist, then __/dev/ram0__ will NOT be fully unmounted if __/dev/ram0__ is used by any process or has any file-system mounted on it. If __/dev/ram0__ is NOT fully unmounted, then __/dev/ram0__ will remain in memory. |
| 1 | perry | 89 | |
| 4 | CraigBox | 90 | 3. Users of __/dev/initrd__ should not depend on the behavior give in the above notes. The behavior may change in future versions of the Linux kernel. |
| 1 | perry | 91 | |
| 92 | !!AUTHOR | ||
| 4 | CraigBox | 93 | The kernel code for device __initrd__ was written by Werner Almesberger <almesber@lrc.epfl.ch> and Hans Lermen <lermen@elserv.ffm.fgan.de>. The code for __initrd__ was added to the baseline Linux kernel in development version 1.3.73. |
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