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Old 02-28-2011, 01:15 PM   #1
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where does the "initrd" source code resides? thank you

After one compiles the kernel.
A "initrd" binary file is created.

Any suggestions?


Old 02-28-2011, 01:56 PM   #2
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The initrd file should be created in /boot. Some distributions (like Debian) now use initramfs instead, which is similar but a different format.
Old 02-28-2011, 01:59 PM   #3
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"man initrd"

INITRD(4)                  Linux Programmer’s Manual                 INITRD(4)

       initrd - boot loader initialized RAM disk

       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  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.

       When booting up with initrd, the system boots as follows:

         1. The boot loader loads the kernel program and /dev/initrd’s contents into memory.

         2. 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.

         3. The kernel then read-write mounts device /dev/ram0 as the initial root file system.

         4.  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.

         5. 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.)

         6. If /linuxrc is not executed or when /linuxrc terminates, the normal root file system is mounted.  (If  /lin-
         uxrc  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.)

         7. 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 behav-
         ior of the kernel is UNSPECIFIED.  See the NOTES section for the current kernel behavior.)

         8. The usual boot sequence (e.g. invocation of /sbin/init) is performed on the normal root file system.

       The following boot loader options when used with initrd, affect the kernel’s boot-up operation:

              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 spec-
              ified with this option will typically be a gzipped file-system image.

              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.

              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  suit-
              able root file-system.

       By default, the kernel’s settings (e.g. set in the kernel file with rdev(8) 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 sys-
       tem,  one has to use the nfs_root_name and nfs_root_addrs boot options to give the NFS settings.  For more infor-
       mation 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/ker-
       nel/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/ker-
       nel/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-dev
       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 for a system with IP number
       and named ’idefix’:
            echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
            echo \
            echo 255 >/proc/sys/kernel/real-root-dev

       Note: The use of /proc/sys/kernel/real-root-dev to change the root file  system  is  obsolete.   See  the  kernel
       source  file  Documentation/initrd.txt  as  well as pivot_root(2) and pivot_root(8) for information on the modern
       method of changing the root file system.

       The main motivation for implementing initrd was to allow for modular kernel configuration at system installation.

       A possible system installation scenario is as follows:

         1.  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.

         2.  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.

         3. The executable /linuxrc loads the necessary modules from the initial root file-system.

         4.  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.)

         5. 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.

         6. The kernel then mounts the normal root file system.

         7. Now that the file system is accessible and intact, the boot loader can be installed.

         8.  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.)

         9. The system is now bootable and additional installation tasks can be performed.

       The key role of /dev/initrd in the above is to re-use the configuration data during normal system operation with-
       out 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 /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-sys-
       tem  partition is not needed at boot time, the system loaded from /dev/initrd can use a dialog and/or auto-detec-
       tion 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 dis-
       tribution can use LOADLIN to directly load /dev/initrd from CD-ROM without the need of any floppies.  The distri-
       bution could also use a LILO boot floppy and then bootstrap a bigger ram disk via /dev/initrd from the CD-ROM.

       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:

               mknod -m 400 /dev/initrd b 1 250
               chown root:disk /dev/initrd
       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.


       chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)

       The documentation file initrd.txt in the kernel source package, the LILO documentation,  the  LOADLIN  documenta-
       tion, the SYSLINUX documentation.

       1.  With  the current kernel, any file systems that remain mounted when /dev/ram0 is moved from / to /initrd con-
       tinue 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.

       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.

       The  kernel code for device initrd was written by Werner Almesberger <> and Hans Lermen <ler->.  The code for initrd was added to the  baseline  Linux  kernel  in  development  version

Linux 2.0                         1997-11-06                         INITRD(4)


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