Recipe 10.10. Creating an initrd Image

10.10.1 Problem

You are using SCSI drives, and you like to load the drivers as modules, because there are so many different SCSI drivers. You want the flexibility to change drives or controllers without rebuilding the kernel every time, and you don't want to fatten your kernel by compiling in every possible driver. But how will the system boot without the SCSI driver built into the base kernel?

10.10.2 Solution

Build an initrd (initialize RAM disk) image with mkinitrd (make initial RAM disk) after you build the kernel. Then add an entry to your bootloader that loads the image.

On both 2.4 and 2.6 kernels, after running make modules_install, run mkinitrd:

# mkinitrd -o /boot/initrd-2.4.25-new-kernel.img

This builds the image and installs it into the /boot directory. Then create your bootloader entries. In GRUB:

title     Kernel  2.4.25, new kernel root      (hd0,0) kernel    /boot/bzImage-2.4.25-new-kernel root=/dev/hda1 ro initrd    /boot/initrd-2.4.25-new-kernel.img

LILO users do this:

image=/boot/bzImage-2.4.22-new-kernel     initrd=/boot/initrd-2.4.25-new-kernel.img     label=Kernel 2.4.22, new kernel     root=/dev/hda1     read-only

Remember to run /sbin/lilo to activate the changes.

10.10.3 Discussion

Be sure to read the mkinitrd man page for your system, as the options differ slightly between the different distributions.

It's perfectly okay to build your SCSI driver into the base kernel; using initrd isn't required.

Most of the major Linux distributions use initrd, so that their stock kernels will boot on most systems without needing every driver in the world to be built in.

10.10.4 See Also

• mkinitrd(8)