15.2, SYSTEMD Daemon Process Introduction

1, the system starts the process review:

Post (power-on self-test, mainly used to check whether the hardware environment that supports system startup is satisfied)---> bootseqence (to load the corresponding boot item device according to the boot order set in the BIOS)---> Bootloader (Load bootloader According to the MBR of the boot device)---> Kernel (load kernel to memory)---> rootfs (Root toggle)---> init (/sbin/init)

Different system init programs are not the same:

CENTOS5:SYSV init//v for version number, Fifth edition

Centos6:upstart

Centos7:systemd

2, Systemd new features introduced:

Implementation of service parallel boot at System boot

Activate processes on demand: this means that processes are activated only when this process is required to run

Support for System State snapshots

The service control logic that is defined based on the dependency relationship.

3. SYSTEMD Core Concept: unit

The unit is identified and configured by its associated configuration file, which mainly contains: system services, listening sockets, saved snapshots, and other unit-related information, which are mainly stored in/usr/lib/systemd/system and/etc/ systemd/system/, as well as the/run/systemd/system/directory.

Common Types of unit:

Unit type	File name extension	Use	Note
Service Unit	. Service	Defining System Services
Taget Unit	. Target	For simulating the run level of INIT implementation
Device Unit	. device	Devices used to define kernel recognition
Mount Unit	. Mount	Used to define file system mount points
Scoket Unit	. scoket	Socket file used to identify inter-process communication
Snapshot unit	. snapshot	For managing system Snapshots
Swap Unit	. Swap	Used to identify swap devices
AutoMount Unit	. automount	File System auto mount point settings
Path unit	. path	Used to define a file or directory in the file system.

4. Key Features of SYSTEMD:

Socket-based activation mechanism, socket and program separation

Bus-based activation mechanism

Device-based activation mechanism

Mechanism of activation of path

System snapshot: Saves the current state information for each unit in a persistent storage device

Backward compatible SYSV init script: Scripts stored in the/etc/init.d/directory

5. Systemd Incompatible Features: (some features incompatible with the old version)

The SYSTEMCTL command is fixed at the same time.

A service that is not initiated by SYSTEMD Systemctl cannot communicate with it, that is, Systemctl cannot control the service.

15.2, SYSTEMD Daemon Process Introduction