INITRDSection: Linux Programmer's Manual (4)
Return to Main Contents
NAMEinitrd - boot loader initialized RAM disk
DESCRIPTIONThe 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.
In the first boot-up phase, the kernel starts up and mounts an initial root file-system from the contents of /dev/initrd (e.g. RAM disk initialized by the boot loader). 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 mounted from a different device.
BOOT-UP OPERATIONWhen booting up with initrd, the system boots as follows:
OPTIONSThe following boot loader options when used with initrd, affect the kernel's boot-up operation:
CHANGING THE NORMAL ROOT FILE SYSTEMBy 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.
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:
echo 0x365 >/proc/sys/kernel/real-root-devFor 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 18.104.22.168 for a system with IP number 22.214.171.124 and named 'idefix':
echo /var/nfsroot >/proc/sys/kernel/nfs-root-name echo 126.96.36.199:188.8.131.52::255.255.255.0:idefix \ >/proc/sys/kernel/nfs-root-addrs echo 255 >/proc/sys/kernel/real-root-dev
USAGEThe main motivation for implementing initrd was to allow for modular kernel configuration at system installation.
A possible system installation scenario is as follows:
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.
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.
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.
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.
CONFIGURATIONThe /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:
mknod -m 400 /dev/initrd b 1 250 chown root:disk /dev/initrdAlso, 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.
SEE ALSOchown(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.
NOTES1. 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.
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.
AUTHORThe kernel code for device initrd was written by Werner Almesberger <email@example.com> and Hans Lermen <firstname.lastname@example.org>. The code for initrd was added to the baseline Linux kernel in development version 1.3.73.
This document was created by man2html, using the manual pages.
Time: 01:37:43 GMT, July 08, 2020