Maintenance boot process
Occasions might arise when a boot is needed to perform special tasks such as installing new or updated software, performing diagnostic checks, or for maintenance. In this case, the system starts from a bootable medium such as a CD-ROM, DVD, tape drive, network, or disk drive.
The maintenance boot sequence of events is similar to the sequence of a normal boot.
- The firmware performs basic testing on the system resources that are required for starting the operating system.
- The firmware checks the user boot list. You can
use the bootlist command to change
the user boot list to suit your requirements. If the user boot list
in non-volatile random access memory (NVRAM) is not valid or if no
valid boot device is found, the default boot list is checked. In either
case, the first valid boot device found in the boot list is used for
starting the system. Note: For a normal boot, the operating system also maintains a default boot list and a user boot list, which are stored in NVRAM. Separate default boot list and user boot list are also maintained for starting the system in maintenance mode.
- 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 initial program load (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 the offset to the entry point to start running when the boot image is in memory.
- The boot image is read sequentially from the boot device into memory, starting at the location specified in NVRAM.
- Control is passed to the kernel, which begins running programs in the RAM file system.
- The ODM database contents determine which devices are present, and the cfgmgr command dynamically configures all devices found, including all disks which are to contain the root file system.
- If a CD-ROM, DVD, tape, or the network is used to boot the system, the rootvg volume group (or rootvg) is not varied on, because the rootvg might not exist (as is the case when installing the operating system on a new system). Network configuration can occur at this time. No paging occurs when a maintenance boot is performed.
At the end of this process, the system is ready for installation, maintenance, or diagnostics.
Note: If the system is started from the hard disk, the rootvg is varied on, the hard disk root file system and the hard disk /usr file system are mounted in the RAM file system, a menu is displayed that allows you to enter various diagnostics modes or single-user mode. If you select single-user mode, you can continue the boot process and enter single-user mode, where the init run level is set to the letter S. The system is then ready for maintenance, software updates, or for running the bosboot command.