Installation Ramdisk Guide

Starting out

• The purpose of the installation ramdisk is to create a small root file system that contains all of the necessary scripts and binaries to automatically install Linux over a network connection. Theoretically, the process should be hands-free after the 'n' key is pressed to start the net-boot procedure. The cluster HOWTO has a nice overview of the process in step 7.3.
• To begin with, create a directory named install-ramdisk, or unpack the compressed tar of our own ramdisk. Any necessary libraries, binaries, and device files can be place in this directory.

How our's works

• Notice first that all of the necessary files have been placed in dev, lib, etc, and sbin. All other directories are either links to these directories or mount points for file systems. This makes the task of managing the files much easier.
• Notice also that sbin/init is now a link to sbin/setup.py, which is a python script. Understanding this script is the key to understanding the installation process. This process will be run instead of init, making setup.py the controlling process. All disk formatting and file copying occurs through this script.
• Some of the files on our ramdisk are no longer being used, but are still included for reference. Among these is rsync, which is demonstrated in commented code in setup.py.

A walkthrough of setup.py

• The script begins by initializing some constants, mounting /proc, and starting the network and portmap. Most of this is accomplished through calls to other programs and scripts in sbin. Note that if you are having problems, you can add a debug shell to the script, remake the romfs image, and poke around a bit. When you are done using the shell, type 'exit' and the script will continue.
• At this point, the script zeroes the partition map and opens pdisk in interactive mode. The partition table for /dev/hda is chosen for editing, an hfs boot partition is created, and the linux swap and root partitions are created. Output from pdisk is written to the screen, and the partitions are formatted.
• After the root file system is mounted, installation can begin. Note the commented code containing an installation method that uses rsync to copy files from the node image on the server to /real-root. For various reasons including an apparently faulty processor on one of our nodes that only choked on rsync (straight out of the "Huh?" files), we have been experimenting with the second form of installation found here which consists of mounting the node image via NFS and using a piped tar command to copy files with permissions intact. The -p option preserves file properties, and the -v option on the second tar prints every copied file to stdout, where it is routed to /real-root/tar-log.
• After moving the log files and setting up environment variables, the hfs utilities are used to format the hfs partition for booting. The bootloader (yaboot) is copied into /boot on the hfs partition along with yaboot.conf and a backup kernel. Yaboot's type is set to 'tbxi' and the /boot is "blessed," making it the boot directory. The Macintosh boot process will now boot from yaboot.
• Once everything is done, the system unmounts file systems and reboots.

Finishing up

• Once you are done, use genromfs to create the ramdisk image and place it in /tftpboot along with yaboot and your netboot kernel. We use a script for convenience. Notice the image can be compressed.

Some tips about yaboot

• Once the system has rebooted and is running, you can mount the boot directory as an hfs file system and change yaboot.conf or update the backup kernel. Yaboot.conf takes the same options as lilo and other bootloaders.