Slackware initrd mini HOWTO by Patrick Volkerding, Sun Jul 12 23:23:24 CDT 2009 This document describes how to create and install an initrd, which may be required to use the 2.6 kernel. Also see "man mkinitrd". 1. What is an initrd? 2. Why to I need an initrd? 3. How do I build the initrd? 4. Now that I've built an initrd, how do I use it? 1. What is an initrd? Initrd stands for "initial ramdisk". An initial ramdisk is a very small Linux filesystem that is loaded into RAM and mounted as the kernel boots, and before the main root filesystem is mounted. 2. Why do I need an initrd? The usual reason to use an initrd is because you need to load kernel modules before mounting the root partition. Usually these modules are required to support the filesystem used by the root partition (ext3, reiserfs, xfs), or perhaps the controller that the hard drive is attached to (SCSI, RAID, etc). Essentially, there are so many different options available in modern Linux kernels that it isn't practical to try to ship many different kernels to try to cover everyone's needs. It's a lot more flexible to ship a generic kernel and a set of kernel modules for it. 3. How do I build the initrd? The easiest way to make the initrd is to use the mkinitrd script included in Slackware's mkinitrd package. We'll walk through the process of upgrading to the generic Linux kernel using the packages found in Slackware's slackware/a/ directory. First, make sure the kernel, kernel modules, and mkinitrd package are installed (the current version numbers might be a little different, so this is just an example): installpkg kernel-generic- installpkg kernel-modules- installpkg mkinitrd-1.3.4-i486-3.txz Change into the /boot directory: cd /boot Now you'll want to run "mkinitrd". I'm using reiserfs for my root filesystem, and since it's an IDE system the reiserfs module will be the only one I need to load: mkinitrd -c -k -m reiserfs This should do two things. First, it will create a directory /boot/initrd-tree containing the initrd's filesystem. Then it will create an initrd (/boot/initrd.gz) from this tree. If you wanted to, you could make some additional changes in /boot/initrd-tree/ and then run mkinitrd again without options to rebuild the image. That's optional, though, and only advanced users will need to think about that. Here's another example: Build an initrd image using Linux kernel modules for a system with an ext3 root partition on /dev/hdb3. Note that you need the mbcache, jbd, and ext3 modules to use ext3: mkinitrd -c -k -m mbcache:jbd:ext3 -f ext3 -r /dev/hdb3 4. Now that I've built an initrd, how do I use it? Now that you've got an initrd (/boot/initrd.gz), you'll want to load it along with the kernel at boot time. If you use LILO for your boot loader you'll need to edit /etc/lilo.conf and add a line to load the initrd. Here's an example section of lilo.conf showing how this is done: # Linux bootable partition config begins image = /boot/vmlinuz-generic- initrd = /boot/initrd.gz root = /dev/hda6 label = Lnx26296 read-only # Linux bootable partition config ends The initrd is loaded by the "initrd = /boot/initrd.gz" line. Just add the line right below the line for the kernel image you use. Save the file, and then run LILO again ('lilo' at the command line). You'll need to run lilo every time you edit lilo.conf or rebuild the initrd. Other bootloaders such as syslinux also support the use of an initrd. See the documentation for those programs for details on using an initrd with them. --------- Have fun!