Red Hat uses loadable kernel modules to dynamically add or remove capabilities in a running kernel. To be able to read an ext3 filesystem for example, the kernel must load the ext3 kernel module.
At boot time the kernel is loaded by a boot loader (GRUB or LILO for example). An initrd image that contains kernel modules needed at boot time is also loaded. If the root ( / ) directory is on an ext3 formatted partition, in order to be able to read from that filesystem the boot kernel must load an initrd image that contains the ext3 kernel module.
If an initrd image is missing or that image does not include suitable kernel modules to access the filesystem on the partition, an error message similar to the following will be seen:
Kernel panic: VFS: Unable to mount root fs on ...
When a new kernel package supplied by Red Hat is installed, a new kernel initrd image is usually created automatically by a post-install script included in the kernel RPM package.
Under some circumstances, an initrd may fail to be created, usually because there is a problem with loopback device or a temporary filesystem is mounted with tmpfs
To manually create initrd image use mkinitrd command.