AIX boot proccess

AIX Boot Process


POST (Power on Self Test) / BIST (Built-in Self Test for AIX)

ROS Initialization (Checks Mother Board using OCS BUMP)

Hardware ROS                                                Software ROS

                                                    3 types:                                                                    

  1. Primary Devices: processor, mother board, RAM
  2. Secondary Devices: HD, CD, Floppy, Tape, magnetic tape etc.
  3. Base Devices: Keyboard, Mouse


  • Checks User Bootlist
  • Locates boot strap code to find bootable devices in User boot list
  • Bootlist is of 2 types:
    • Bootlist in Normal Boot
    • Bootlist in Service Mode
  • Bootlist in Normal Boot & Service mode each is of 2 types:
    • Default Bootlist in ROS
    • User Bootlist in NVRAM




User BootList: 3 Scenarios

  1. Available in NVRAM and is valid: loads Boot device from the user Bootlist found
  2. Available in NVRAM AND IS INVALID: Checks in Default Bootlist in ROS and loads the boot device
  3. Unavailable: keep checking the adapters and devices on BUS till valid bootlist is found to load the boot devices.

Once a Valid boot device is found

PSN (Program Sector Number) is checked

PSN gets loaded in IPL (Initial Program Load)

IPL contains information about the boot image

Boot Image is made up of:

  • Kernel
  • RAM FS
  • Device Information (ODM)
  • RC Boot (it’s a directory)

Control goes over to the Kernel

AIX Kernel gets initiated and RAM FS is created

Kernel will start the INIT process with Process ID = 1

INIT starts RC.Boot1 (Boot Phase 1)

  1. Copies ODM information from BLV into RAM FS (Command: # restbase)
  2. Starts sys, bus, SCSI, LVM, RVG configuration methods and updates data present in ODM in RAM FS (Command: # cfgmgr -f). cfgmgr command will check for all plug and play devices.
  3. Determines the last boot device (Command: # bootinfo -b)

Now the system is ready for installation, maintenance or diagnostic

INIT starts RC.Boot2 (Boot Phase 2)

  1. Activates the Root VG information available in ODM in RAM FS (Command: # ipl_varyon)
  2. Mounts HD FS (Hard disk FS) in RAM FS. All 10 partitions gets loaded
  3. Copies ODM from RAM FS into HD FS and scans the file system (Command: # fsck -f)
  4. Delete RAM FS

Up to this no console is available; boot progress is checked through LED display. All boot information is written in log file, can be checked by running the following command:

# alog –t boot -o

INIT will execute the processes defined in /etc/inittab

One of the processes is RC.Boot3 (Boot Phase 3)

  1. Checks stale partitions in VG and mounts temp in HD FS (Command: # syncvg)
  2. Synchronize VG information available in ODM (Command: # savebase)
  3. Initialize the console (Command: # cfgcon)

Kernel will start all the Background daemon services present in /etc/inittab

LED display is OFF

Login Display will appear

End of Boot process, Root FS is mounted

Init reads /etc/inittab

If default entry does not exists, init will ask user to enter the Run level from system console

If /etc/inittab does not exists, system will go into maintenance mode

Init re-reads /etc/inittab every 60 seconds


Points To Remember:

  1. ODM: Object Database Manager: is a database that has complete device configuration and informational data of the entire OS.

Information stored in ODM is:

Network Configuration

LVM Configuration

PV VG LV Configuration

Device Information

Smit menus, screens and commands

Hardware information

  1. OCS BUMP stands doe On Chip Sequencer and Built in Micro processor
  2. S: All information available here is gained and compiled from various RED HAT IBM Books & PDFs.

This is an effort to make understanding of AIX Booting Process simpler and user friendly.

All thoughts and reflections are welcomed.

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