System Boot process in AIX

                                                     System Boot process in AIX

Most users perform a hard disk boot when starting the system for general operations. The system finds all information necessary to the boot process on its disk drive.

When the system is started by turning on the power switch (a cold boot) or restarted with the reboot or shutdown commands (a warm boot), a number of events must occur before the system is ready for use. These events can be divided into the following phases: 

                •    ROS kernel init phase

The ROS kernel resides in firmware.

Its initialization phase involves the following steps:

1.    The firmware checks to see if there are any problems with the system board. Control is passed to ROS, which performs a power-on self-test (POST).

2.    The ROS initial program load (IPL) checks the user boot list, a list of available boot devices. This boot list can be altered to suit your requirements using the bootlist command. If the user boot list in non-volatile random access memory (NVRAM) is not valid or if a valid boot device is not found, the default boot list is then checked. In either case, the first valid boot device found in the boot list is used for system startup. If a valid user boot list exists in NVRAM, the devices in the list are checked in order. If no user boot list exists, all adapters and devices on the bus are checked. In either case, devices are checked in a continuous loop until a valid boot device is found for system startup.

Note: The system maintains a default boot list that is stored in NVRAM for normal mode boot. A separate service mode boot list is also stored in NVRAM, and you should refer to the specific hardware instructions for your model to learn how to access the service mode boot list.

3.    When a valid boot device is found, the first record or program sector number (PSN) is checked. If it is a valid boot record, it is read into memory and is added to the IPL control block in memory. Included in the key boot record data are the starting location of the boot image on the boot device, the length of the boot image, and instructions on where to load the boot image in memory.

4.    The boot image is read sequentially from the boot device into memory starting at the location specified in NVRAM. The disk boot image consists of the kernel, a RAM file system, and base customized device information.

5.    Control is passed to the kernel, which begins system initialization.

6.    The kernel runs init, which runs phase 1 of the rc.boot script.

When the kernel initialization phase is completed, base device configuration begins.

                                  •    Base device configuration phase

The init process starts the rc.boot script. Phase 1 of the rc.boot script performs the base device configuration.

Phase 1 of the rc.boot script includes the following steps:

1.    The boot script calls the restbase program to build the customized Object Data Manager (ODM) database in the RAM file system from the compressed customized data.

2.    The boot script starts the configuration manager, which accesses phase 1 ODM configuration rules to configure the base devices.

3.    The configuration manager starts the sys, bus, disk, SCSI, and the Logical Volume Manager (LVM) and rootvg volume group configuration methods.

4.    The configuration methods load the device drivers, create special files, and update the customized data in the ODM database.

                                  •    Booting the system

Use these steps to complete the system boot phase.

1.    The init process starts phase 2 running of the rc.boot script. Phase 2 of rc.boot includes the following steps:

          a.    Call the ipl_varyon program to vary on the rootvg volume group.

          b.    Mount the hard disk file systems onto their normal mount points.

          c.    Run the swapon program to start paging.

          d.    Copy the customized data from the ODM database in the RAM file system to the  ODM database in the hard disk file system.

          e.    Exit the rc.boot script.

 After phase 2 of rc.boot, the boot process switches from the RAM file system to the hard disk root file system.