Linux-based CentOS system start-up process

CentOS system Start-up process

Components of the Linux system: Kernel + Root file system

Kernel: Process management, memory management, network protocol stack, file system, drivers, security features

Ipc:inter Process Communication

Message Queuing, Semerphor, SHM

Socket

The system environment in operation can be divided into two tiers: kernel space, user space

User space: Application (process or thread)

Kernel Space: Kernel code (System call)

Kernel design Genre:

Single Core design: Integrate all functions into the same program;

Linux

Microkernel Design: Each function is implemented using a separate subsystem;

Windows, Solaris

Linux kernel Features:

Support modularity:. KO (Kernel object)

Supports dynamic loading or unloading of modules during runtime;

Part:

Core file:/boot/vmlinuz-version-release

RAMDisk

CentOS 5:/boot/initrd-version-release.img

CentOS 6,7:/boot/initramfs-version-release.img

Module file:/lib/modules/version-release

The start-up process for the CentOS system:

Post: Power-on self-test;

Rom:cmos

Bios:basic Input and Output System

Rom+ram

Boot Sequence:

In order to find the boot device, the first device with a boot program is the device to be used for this launch;

Bootloader: Boot loader, program;

Windows:ntloader

Linux:

Lilo:liinux LOader

Grub:grand Uniform Bootloader

GRUB 0.x:grub Legacy

GRUB 1.X:GRUB2

Function: Provides a menu that allows the user to select the system to be launched or a different kernel version, to load the user-selected kernel into a specific space in RAM, unzip, expand, and then transfer control of the system to the kernel;

Mbr:master Boot Record

512bytes:

446bytes:bootloader

64bytes:fat

2bytes:55aa

Grub:

bootloader:1st Stage

Partition:filesystem driver, 1.5 stage

Partition:/boot/grub, 2nd Stage

Note: uefi,gpt

Kernel:

Self-initialization:

Detect all the hardware devices that can be identified;

Load the hardware driver; (possibly with ramdisk load driver)

Mount the root file system in read-only mode;

The first application running user space:/sbin/init

Types of init programs:

CentOS 5-:SYSV Init

Configuration file:/etc/inittab

CentOS 6:upstart

Configuration file:/etc/inittab

/etc/init/*.conf

CentOS 7:SYSTEMD

Configuration files:/usr/lib/systemd/system/,/etc/systemd/system/

RAMDisk

One of the features of the Linux kernel: use buffering and caching to speed up file access on disk;

RAMDisk-Ramfs

CentOS 5:INITRD

Tool Program: MKINITRD

CentOS 6,7:initramfs

Tool Program: Dracut, MKINITRD

System initialization process (Kernel level): POST--Bootsequence (BIOS)--BootLoader (MBR)--Kernel (RAMDisk)--Rootfs (readonly)-- >/sbin/init ()

/sbin/init:

CentOS 5:SYSV Init

Operating level: a mechanism set up for the purpose of operating or maintaining the system;

0-6:7 levels;

0: Shutdown, shutdown

1: Single user mode, root user, no authentication, maintenance mode;

2, multi-user mode (multi user), will start the network function, but will not start NFS; maintenance mode;

3. Multi-user mode (Mutli user), full function mode, text interface;

4, Reserved level: Currently no special use purposes, but the habit of using the same 3-level function;

5, multi-user mode (multi user), full function mode, graphical interface;

6, restart, reboot

Default Level: 3, 5

Level switch: Init #

Level view:

Who-r

RunLevel

Configuration file:/etc/inittab

Each row defines an action and the process that corresponds to it

Id:runlevels:action:process

ID: The identifier of a task;

Runlevels: The level at which this task is started; #,### can also be empty, indicating all levels;

Action: Under what conditions to start this task;

Process: task;

Action

Wait: Executes once while waiting to switch to the level at which the task is located;

Respawn: Once this task is terminated, it will be restarted automatically;

Initdefault: Sets the default RunLevel, at which time the process is omitted;

Sysinit: Set the system initialization mode, here is generally designated/etc/rc.d/rc.sysinit script;

For example:

Id:3:initdefault:

Si::sysinit:/etc/rc.d/rc.sysinit

L0:0:WAIT:/ETC/RC.D/RC 0

L1:1:WAIT:/ETC/RC.D/RC 1

............

L6:6:WAIT:/ETC/RC.D/RC 6

means to start or close the service that the service script under the/etc/rc.d/rc3.d/directory controls;

K*: The service to be stopped, k##*, priority, the smaller the number, the more priority to shut down, the dependent services are first closed, and then closed to be dependent;

S*: The service to be started, s##*, priority, the smaller the number, the more priority to start, the dependent service starts first, and the dependent service starts;

RC script: Accepts a run-level number as a parameter;

Script framework:

for SRV in/etc/rc.d/rc#.d/k*; Do

$srv stop

Done

for SRV in/etc/rc.d/rc#.d/s*; Do

$srv start

Done

/etc/init.d/* (/etc/rc.d/init.d/*) Script execution method:

#/etc/init.d/srv_script {Start|stop|restart|status}

# service Srv_script {start|stop|restart|status}

Chkconfig command: Control/etc/init.d/the startup or shutdown status of each service script at all levels;

View: Chkconfig--list [Name]

Added: Chkconfig--add name

One of the script definition formats for the service that can be added:

#!/bin/bash

#

# chkconfig:lll nn nn

# Description:

Delete: Chkconfig--del name

To modify the specified link type:

Chkconfig [--level levels] name <on|off|reset>

--level levels: Specifies the level to be controlled; default is 2345;

Note: At normal level, a service s99local that was last started is not linked to a script under/ETC/INIT.D, but is linked to/etc/rc.d/rc.local (/etc/ rc.local) script, so that a program that is inconvenient or does not need to be written as a service script is placed in this script file directly when it is expected to run automatically.

Tty1:2345:respawn:/usr/sbin/mingetty tty1

... ...

Tty6:2345:respawn:/usr/sbin/mingetty Tty6

(1) Mingetty will invoke the login program;

(2) Open the virtual terminal program in addition to Mingetty, there are such as Getty, etc.;

System Initialization script:/etc/rc.d/rc.sysinit

(1) Set the host name;

(2) Set welcome information;

(3) Activate Udev and SELinux;

(4) Mount all file systems defined in the/etc/fstab file;

(5) Detecting the root file system and re-mounting the root file system in read-write mode;

(6) Set the system clock;

(7) Set kernel parameters according to/etc/sysctl.conf file;

(8) activating LVM and soft raid devices;

(9) Activate swap device;

(10) Loading the drivers for additional devices;

(11) Cleaning operation;

Summary (User space Start-up process):/sbin/init (/etc/inittab)

Set the default run level-run the system initialization script, complete the system initialization--close the service that needs to be stopped at the corresponding level, start the service that needs to be turned on at the corresponding level, and set the login terminal [--> to start the graphics terminal]

CentOS 6:

Init program: Upstart, but still for/sbin/init, its configuration file:

/etc/init/*.conf,/etc/inittab (for defining the default RunLevel only)

Note: *.conf is a upstart-style configuration file;

Rcs.conf

Rc.conf

Start-ttys.conf

CentOS 7:

Init program: systemd, config file:/usr/lib/systemd/system/*,/etc/systemd/system/*

Fully compatible with the SYSV scripting mechanism; Therefore, service commands are still available; however, it is recommended to use the SYSTEMCTL command to control services;

# systemctl {start|stop|restart|status} Name[.service]

Blog assignment: CentOS system start-up process;

Linux-based CentOS system start-up process