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AIX Version 4.3 System Management Guide: Operating System and Devices

Understanding the RAM File System

The RAM file system, part of the boot image, is totally memory-resident and contains all programs that allow the boot process to continue. The files in the RAM file system determine the type of boot.

A service boot RAM file system might not have the logical volume routines, since the rootvg volume group may not need to be varied on. During a hard disk boot, however, it is desirable that the rootvg volume group be varied on and paging activated as soon as possible. Although there are differences in these two boot scenarios, the structure of the RAM file system does not vary to a great extent.

The init command on the RAM file system used during boot is actually the ssh (simple shell) program. The ssh program controls the boot process by calling the rc.boot script. The first step for rc.boot is to determine from what device the machine was booted. The boot device determines which devices should be configured on the RAM file system. If the machine is booted over the network, the network devices need to be configured so that the client's file systems can be remotely mounted. In the case of a tape or CD-ROM boot, the console is configured to display the BOS install menus. After the rc.boot script finds the boot device, then the appropriate configuration routines are called from the RAM file system. The rc.boot script itself is called twice by the ssh program to match the two configuration phases during boot. A third call to rc.boot occurs during a disk or a network boot when the real init command is called. There is a rc.boot stanza in the inittab file that does the final configuration of the machine.

The RAM file system for each boot device is also unique due to the various types of devices to be configured. There is a prototype file associated with each type of boot device. The prototype file is a template of files making up the RAM file system. The mkfs command is used by the bosboot command to create the RAM file system using the various prototype files. See the bosboot command for more details.

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