Linux Initial RamDisk (Overview )

Summary:  
The Linux initial RAM disk (initrd) is a temporary root file system that is mounted during system boot to support the two-state boot process. The initrd contains various executables and drivers that permit the real root file system to be mounted, after which the initrd RAM disk is unmounted and its memory freed. In many embedded Linux systems, the initrd is the final root file system. This article explores the initial RAM disk for Linux 2.6, including its creation and use in the Linux kernel.

The special file /dev/initrd is a read-only block device. This device is a RAM disk that is initialized (e.g., loaded) by the boot loader before the kernel is started. The kernel then can use /dev/initrd's contents for a two-phase 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 operation

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 the 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 /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.)

7. If the normal root file system has a directory /initrd, the device /dev/ram0 is moved from / to /initrd. Otherwise if the directory /initrd does not exist, the 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.)

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

  What's an initial RAM disk?

 The initial RAM disk (initrd) is an initial root file system that is mounted prior to when the real root file system is available. The initrd is bound to the kernel and loaded as part of the kernel boot procedure. The kernel then mounts this initrd as part of the two-stage boot process to load the modules to make the real file systems available and get at the real root file system.
The initrd contains a minimal set of directories and executables to achieve this, such as the insmod tool to install kernel modules into the kernel.
In the case of desktop or server Linux systems, the initrd is a transient file system. Its lifetime is short, only serving as a bridge to the real root file system. In embedded systems with no mutable storage, the initrd is the permanent root file system. This article explores both of these contexts.



Structure of the initrd
The initrd image contains the necessary executables and system files to support the second-stage boot of a Linux system.
Depending on which version of Linux you're running, the method for creating the initial RAM disk can vary. Prior to Fedora Core 3, the initrd is constructed using the loop device. The loop device is a device driver that allows you to mount a file as a block device and then interpret the file system it represents. The loop device may not be present in your kernel, but you can enable it through the kernel's configuration tool (make menuconfig) by selecting Device Drivers > Block Devices > Loopback Device Support. You can inspect the loop device as follows (your initrd file name will vary):

Listing 1. Inspecting the initrd 

 
# mkdir temp ; cd temp
# cp /boot/initrd-2.6.14.2.img initrd-2.6.14.2.img.gz
# gunzip initrd-2.6.14.2.img.gz
# cpio -i --make-directories < initrd-2.6.14.2.img
#  

The result is a small root file system, as shown in Listing 3. The small, but
necessary, set of applications are present in the ./bin directory, including
nash (not a shell, a script interpreter), insmod for loading kernel modules, and
lvm (logical volume manager tools). 

Listing 3. Default Linux initrd directory structure

# ls -la
#
drwxr-xr-x  10 root root    4096 May 7 02:48 .
drwxr-x---  15 root root    4096 May 7 00:54 ..
drwxr-xr-x  2  root root    4096 May 7 02:48 bin
drwxr-xr-x  2  root root    4096 May 7 02:48 dev
drwxr-xr-x  4  root root    4096 May 7 02:48 etc
-rwxr-xr-x  1  root root     812 May 7 02:48 init
-rw-r--r--  1  root root 1723392 May 7 02:45 initrd-2.6.14.2.img
drwxr-xr-x  2  root root    4096 May 7 02:48 lib
drwxr-xr-x  2  root root    4096 May 7 02:48 loopfs
drwxr-xr-x  2  root root    4096 May 7 02:48 proc
lrwxrwxrwx  1  root root       3 May 7 02:48 sbin -> bin
drwxr-xr-x  2  root root    4096 May 7 02:48 sys
drwxr-xr-x  2  root root    4096 May 7 02:48 sysroot
#

Of interest in Listing 3 is the init file at the root. This file, like the traditional

Linux boot process, is invoked when the initrd image is decompressed into the RAM disk. 

Tools for creating an initrd 

 Let's now go back to the beginning to formally understand how the initrd image is constructed in the first place. For a traditional Linux system, the initrd image is created during the Linux build process. Numerous tools, such as mkinitrd, can be used to automatically build an initrd with the necessary libraries and modules for bridging to the real root file system. The mkinitrd utility is actually a shell script, so you can see exactly how it achieves its result. There's also the YAIRD (Yet Another Mkinitrd) utility, which permits customization of every aspect of the initrd construction.

CPIO:
Using the cpio command, you can manipulate cpio files. Cpio is also a file format that is simply a concatenation of files with headers. The cpio file format permits both ASCII and binary files. For portability, use ASCII. For a reduced file size, use the binary version.

#############
 Steps to push a module into initrd

1. Uncompress the initrd module
2. Traverse to the Modules locations
3. Place the module in the Require segment of the folder
4. Pack it back, test it.








Get rid of the Floppy Drive Errors:-

Error Message while Vxrouter Booting.
end_request: I/O error, dev fd0, sector 0
end_request: I/O error, dev fd0, sector 0
end_request: I/O error, dev fd0, sector 0

1. Uncompress the initrd module
2.The below is the kernel object for floppy
rm -f /lib/modules/2.6.26-1-486-vyatta/kernel/drivers/block/floppy.ko
3. Compress the initrd module
4. Boot the system... You are done... No more Floppy drive Errors  on the Screen