What is initrd and How to rebuild the initial ramdisk image

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.

initrd provides the capability to load a RAM disk by the boot loader. This RAM disk can then be mounted as the root filesystem and programs can be run from it. Afterwards, a new root file system can be mounted from a different device. The previous root (from initrd) is then moved to a directory and can be subsequently unmounted.

How initrd works

initrd provides the capability to load a RAM disk by the boot loader. This RAM disk can then be mounted as the root file system and programs can be run from it. Afterwards, a new root file system can be mounted from a different device. The previous root (from initrd) is then moved to a directory and can be subsequently unmounted.

initrd is mainly designed to allow system startup to occur in two phases, where the kernel comes up with a minimum set of compiled-in drivers, and where additional modules are loaded from initrd.

Operation Flow:

When using initrd, the system typically boots as follows,

1. the boot loader loads the kernel and the initial RAM disk
2. the kernel converts initrd into a “normal” RAM disk and frees the memory used by initrd
3. if the root device is not /dev/ram0, the old (deprecated) change_root procedure is followed. see the “Obsolete root change mechanism” section below.
4. root device is mounted. if it is /dev/ram0, the initrd image is then mounted as root
5. /sbin/init is executed (this can be any valid executable, including shell scripts; it is run with uid 0 and can do basically everything init can do).
6. init mounts the “real” root file system
7. init places the root file system at the root directory using the pivot_root system call
8. init execs the /sbin/init on the new root filesystem, performing the usual boot sequence
9. the initrd file system is removed

Note

That changing the root directory does not involve unmounting it. It is therefore possible to leave processes running on initrd during that procedure. Also note that file systems mounted under initrd continue to be accessible.

How to rebuild the initial ramdisk image

You should need ramdisk if you added new hardware devices such as SCSI or FibreChannel controller to your server as the ramdisk contains the necessary modules (i.e. drivers) to initialize hardware driver. If you modified the /etc/modprob.conf (or modules.conf) file then you need to execute special script called mkinitrd.

The mkinitrd script constructs a directory structure that can serve as an initrd root file system. It then generates an image containing that directory structure using mkcramfs, which can be loaded using the initrd mechanism. The kernel modules for the specified kernel version will be placed in the directory structure. If version is omitted, it defaults to the version of the kernel that is currently running.

Find out your kernel version:

# uname -r
2.6.32

Make backup of existing ram disk:

# cp /boot/initrd.$(uname -r).img /root

To create initial ramdisk image type following command as the root user:

# mkinitrd -o /boot/initrd.$(uname -r).img $(uname -r)
# ls -l /boot/initrd.$(uname -r).img

You may need to modify grub.conf to point out to correct ramdisk image, make sure following line existing in grub.conf file:

initrd /boot/initrd.img-2.6.15.4.img

When the system boots using an initrd image created by mkinitrd command, the linuxrc will wait for an amount of time which is configured through mkinitrd.conf, during which it may be interrupted by pressing ENTER. After that, the modules specified in will be loaded.