Kernel Panic - Can't find root filesystem / error mounting /dev/root

Question

From here: http://fedoraproject.org/wiki/Common_kernel_problems#Can.27t_find_root_filesystem_.2F_error_mounting_.2Fdev.2Froot

A lot of these bugs end up being a broken initrd due to bugs in mkinitrd.

Get the user to attach their initrd for their kernel to the bz, and also their /etc/modprobe.conf, or have them examine the contents themselves if they are capable of that.

Picking apart the initrd of a working and failing kernel and doing a diff of the init script can reveal clues. To take apart an initrd, do the following ..

mkdir initrd   
 cd initrd/   
 gzip -dc /boot/initrd-2.6.23-0.104.rc3.fc8.img | cpio -id 

I wish to understand what exactly is being done here.
What has initrd to do with anything?
Where are we supposed to create the directory initrd?

Answer 1

An initrd (short for “initial RAM drive”) is a filesystem that's mounted when the Linux kernel boots, before the “real” root filesystem. This filesystem is loaded into memory by the bootloader, and remains in memory until the real boot. The kernel executes the program /linuxrc on the initrd; its job is to mount the real root, and when /linuxrc terminates the kernel runs /sbin/init.

A bug somewhere in the initrd can explain why the system doesn't boot. So the document you link to recommends that you compare your initrd with an official one if you have trouble booting.

In the provided instructions, initrd is just some temporary directory, you can call it anisha_initrd or fred if you like. The initrd is stored in the file /boot/initrd-SOMETHING.img as a gzipped cpio archive; the instructions unpack that archive in the temporary directory you created. After unpacking, you can compare it with an official initrd (unpack the official initrd and run a command like diff -ru /path/to/official_initrd /path/to/anisha_initrd).

Answer 2

The steps you referred to describe how to examine an initrd image, i.e., how to unpack (via gzip and cpio) the initrd-2.6.23-0.104.rc3.fc8.img file into a directory (initrd/ here, created anywhere, with mkdir). It's just a step towards finding the problem, no solution in itself.