101.2 Boot The System

• Provide common commands to the boot loader and options to the kernel at boot time
• Demonstrate knowledge of the boot sequence form BIOS to boot completion
• Understanding of SysVinit and system
• Awareness of Upstart
• Check boot events in the log file
• The following is a partial list of the used files, terms and utilities:
  • dmesg
  • BIOS
  • bootloader
  • kernel
  • initramfs
  • init
  • SysVinit
  • system

Common Boot loader commands and options to the kernel at boot time

• GRUB configuration
  • View and select from all available kernels
  • Multi-boot support
  • /etc/default/grub
  • grub2 -mkconfig -o  /boot/grub2/grub.cfg
    • Generates new GRUB 2 config
    • Updates config file
• CentOS installation
  • Verify BIOS settings
  • Hardware Compatibility
• Boot Parameters
  • Interactive or persistent commands
  • cat /proc/cmdline
  • Minimal parameters
    • root=
    • ro
• Kernel Parameters
  • quiet (suppress all non-critical kernel messages)
  • log buf len (kernel log buffer size – used by dmesg)
  • modprobe (view and configure kernel modules)
  • crashkernel (save copy of memory for debugging use)
• Ramdisk parameters
  • Loaded into memory by GRUB
  • Initial ramdisk runs /sbin/init script to read boot parameters
  • rdinitdebug (causes init script to print each line during execution)
• Mounting root filesystem (/) during boot
  • May require filesystem module
  • May require hardware drivers
    • HW RAID
    • SAN HBA
    • SCSI

Demonstrate knowledge of the boot sequence from BIOS to boot completion

• System BIOS Tasks
  • Power-On Self Test (POST)
  • Initial hardware setup and configuration
  • Hardware adapter initialized
    • (SCSI, SAN HBA, RAID)
• First stage GRUB is loaded from MBR
  • Stage 1.5 GRUB provides filesystem drivers
  • Stage 2 GRUB is loaded
• Second stage GRUB
  • Reads config file or displays the GRUB command prompt
  • Initial Ramdisk is loaded
  • Linux kernel is loaded from hard drive and decompressed
  • Kernel is executed with command line arguments
• Linux Kernel
  • Hardware is initialized and configured using drivers in kernel
  • initramfs is decompressed and mounted
  • init script runs from initramfs
  • init script loads kernel modules and mounts root filesystem
  • Executes /sbin/init from the mounted root filesystem

Awareness of upstart

• Upstart is an event-based replacement for the init daemon
• It is asynchronous and compatible with sysvinit
• Configuration files
  • /etc/inittab
  • /etc/init/*

Check boot events in the log file

• Boot events can be found in several locations
  • /var/log/boot.log
  • /var/log/messages
  • /var/log/dmesg
• Errors are usually more interesting than successful events…