AIX Boot Process

The AIX boot process is a masterclass in enterprise engineering, transitioning from raw hardware checks to a fully functional multi-user environment. Understanding this sequence is vital for any administrator tasked with troubleshooting or optimizing IBM Power Systems.

The AIX Boot Sequence: Step-by-Step
The journey from "power on" to "login prompt" occurs in four primary phases:

1. Hardware Initialization (POST)
The moment the power button is pressed (or a remote start is triggered via the HMC), the system performs a Power-On Self Test (POST). The Service Processor (FSP) checks the health of processors, memory, and I/O adapters. If everything is green, the system proceeds to the firmware stage.

2. Initial Program Loader (IPL) Phase
This is where the transition from hardware to software begins.
The Bootlist: The firmware checks the defined bootlist to find a valid boot device (SAN, internal disk, or network).
BLV (Boot Logical Volume): The system locates the hd5 partition, which contains the boot image.
Kernel Loading: The AIX kernel and a temporary file system (RAMFS) are loaded into memory. Control is handed over to the kernel.

3. Configuration & rc.boot
The kernel starts the init process, which executes the crucial /sbin/rc.boot script. This script runs in three distinct stages:
Stage 1: Base device configuration (disks, buses) and the mounting of the root file system (/).
Stage 2: The system transitions from the RAM disk to the actual hard disks. The paging space is activated.
Stage 3: The rest of the "defined" devices are configured, and the system prepares for the full multi-user environment.

4. The Init Phase (/etc/inittab)
The init process (PID 1) takes over and reads the /etc/inittab file. This file acts as the "to-do list" for the OS, directing it to:
Set the default run level.
Initialize the System Resource Controller (SRC).
Execute networking scripts like rc.tcpip and rc.nfs.
Launch the getty process to provide the final login prompt.

Key Management Files & Tools

Component	Function
bootlist	Command used to view or set the boot device order.
bosboot	Creates the boot image (BLV) on the boot disk.
IPL	The "Initial Program Load"—the mainframe term for booting.
LED codes	3 or 4-digit codes displayed on the server (or HMC) indicating boot progress or errors.
SMIT	The System Management Interface Tool, used to configure boot settings via menu.

Troubleshooting Tip: Maintenance Mode
If the system hangs at a specific LED code (like 0552, 0554, or 0556), it usually points to a corrupted filesystem or a failure to mount the root Volume Group. In these cases, you can boot into Maintenance Mode (Service Mode) using a diagnostic CD or the HMC to run fsck on your logical volumes.