Tuesday, 30 April 2013

Different Stages of Linux Boot Process

Here checks how linux works starting at power on, and finally reaching the desktop.

1. HardWare Boot(power on):
     BIOS Initialization
     Boot Device Selection / Boot Loder initialization

When it power on it will first check is there anything wrong from the time it went off, In a system this is performed by BIOS, which is stored in CMOS chip.

BIOS acts as an interface between H/W and S/W.

The BIOS is responsible of identifying, checking and initializing system devices such as graphics cards, hard disks, etc, to do this the BIOS makes a POST .

At the end of POST a boot device is selected from a list of detected boot devices (CD-ROM, USB drive, hard disk or floppy disk).
While POST it indicate any errors happened in the device, Fatal errors & Non-Fatal Errors

Fatal: SMPS,HD etc --- Continuous beep sound ---- It will distrube booting process

Non-Fatal: keyboard,mouse --- small beep sound ---- this will not disturb booting process

The BIOS reads and execute the 1st physical sector of the chosen boot media on the system, usually it is the 1st 512 bytes of HD.

this 512 bytes are divided into 3

first 446 bytes containing Booting info(MBR)
second 64 bytes contains partition info
last 2 bytes containing Bootable Media check --- it checks booting devices

2. OS Boot:

     Boot loader
     MBR [Master Boot Recoder]
Boot loader is responsible for loading and starting OS when system starts.

First section is the grub(grant unified boot loader), the grub is in the first 512 bytes of the HD.

Master Boot Recoder[MBR] contains info. about the OS only. The boot loader invoked by either, BIOS passes control to aninitial program loader(IPL) installed within MBR.

IPL in the first 446 bytes of MBR, the taks of the IPL is to locate and load a second stage boot loader for further booting.


  GRUB stands for Grand Unified Bootloader

The grub info are stored in /boot/grub/grub.conf -- this is the second stage boot loader.

If you have multiple kernel images installed on your system, you can choose which one to be executed.

GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.

GRUB just loads and executes Kernel and initrd images

default=0 --- 0 means first kernal will load, 

         if there is 1 means 2nd lying kernal will load

timeout=3 --- loading within this much second


hidden menu

title Redhat enterprise linux server(2.6.18-53.el5)
          ---- in which name OS will load

root (hd0,0) ---- first HD, first partition

kernel /vmlinuz-2.6.18-53.el5 ro root=LABEL=/ rhgb quiet
  ------ kernel with this name(/vmlinuz-2.6.18-53.el5) will
  load and it will mount root file system as read only.

initrd /initrd-2.6.18-53.el5.img ---- this is the kernal
           module which is need to mount the / If we need
           to mount / we need kernal if there is no
           / we can't boot the kernal, so the need
              of initrd image.

initrd ---- The linux kernel needs to mount the root file system. To do so it typically needs access to modules. The kernel cannot mount the root file system without the modules but the modules are in root file system. Then initrd is the solution it is a part of typical grub specification in grub.conf.

initrd indicates that the initial ramdisk file called initrd /initrd-2.6.18-53.el5.img contains the needed modules for mounting root file system.

4. Kernel Initialization
-- The boot loader first loads the kernal. Mounts the root file system as specified in the “root=” in grub.conf

-- Kernel executes the /sbin/init program.

-- initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted.It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.
     Kernal boot time functions are,

  1. Device Detection,Partation Detection
  2. Device Driver Initialization
  3. Mount root file system read only
  4. load initial process ie, init

First device drivers are called and will attempt to locate their corresponding devices. After all the essentials drivers are loaded, the kernal will mount the root file system read only. then the first process is loaded(init process) and control is passed from kernal to that process. ]

5. initialization of init

init reads its configuration in /etc/inittab

this file shows how init should setup the system in every run level,

Runlevel 0: -- shutdown
Runlevel 1:  single user mode(rescue operations are done from here), s/emergency mode in this mode root file system is read only mode.for file system modification, 

    use this e2fsck in this level
Runlevel 2: multi user mode without network
Runlevel 3: multi user mode with network 
Runlevel 4: Undefined, reserved for local use (GUI mode for

                  Slackware only)
Runlevel 5: graphical user interface(GUI)mode,full support
Runlevel 6: reboot

Depending on your default init level setting, the system will execute the programs from one of the following directories.
Run level 0 – /etc/rc.d/rc0.d/
Run level 1 – /etc/rc.d/rc1.d/
Run level 2 – /etc/rc.d/rc2.d/
Run level 3 – /etc/rc.d/rc3.d/
Run level 4 – /etc/rc.d/rc4.d/
Run level 5 – /etc/rc.d/rc5.d/
Run level 6 – /etc/rc.d/rc6.d/

Under the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and K.

Programs starts with S are used during startup.S for startup.
Programs starts with K are used during shutdown.K for kill.

Some system services are running from this files as well

sets kernal prameters in /etc/sysctl.conf
sets system clock
enable swap partations
sets host name
root file system check and remount
activate raid and lvm devices
enable disk quotas
check and mount other file systems

/etc/rc.d/init.d ----- init file which contains script for system services.

No comments:

Post a Comment