SYSTEMD initialization Process

SYSTEMD Initialization Process

Linux operating system start-up process starts from the BIOS → enter "Boot Loader" → load kernel → kernel initialization → start the initialization process , the initialization process as the first process of the system, it needs to complete the relevant initialization work, to provide users with the appropriate working environment.

Red Hat RHEL7 systems have replaced the familiar initialization process system V initand formally adopted the new initialization process SYSTEMD. The initialization process SYSTEMD uses the concurrency boot mechanism, so the boot speed has been improved a little. Although Systemd has shown an advantage, there have been calls for resistance:

Slot 1: Lennart Poettering, the author, worked for Red Hat, making fans of other systems uncomfortable.

The 2:SYSTEMD can only run under the Linux system, abandoning BSD users.

Slot 3: Take over the work of services such as SYSLOGD, Udev, Cgroup and so on, no longer willing to do only the initialization process.

Spit 4: Use SYSTEMD after the RHEL7 system changes too large, reference documentation is not much, embarrassed us ah!

No matter how the groove, since the Red Hat RHEL7 system chose systemd, the original inittab will no longer work, there is no "runlevel" concept, Linux system startup to do a lot of initialization work-such as Mount file system and swap partition, Start all kinds of process services and so on, these can be seen as a unit, the analysis of the NFS service unit configuration file it:

[[email protected] ~]# cat/etc/systemd/system/nfs.target.wants/nfs-lock.service[Unit]Description=nfs file Locking service.//indicates that the Rpcbind service must already be running before the NFS service is started. Requires=rpcbind.service Network.targetafter=network.target Named.service rpcbind.servicebefore= Remote-fs-pre.target[Service]type=forkingstandarderror=syslog+consoleenvironmentfile=-/etc/sysconfig/ nfs//commands to be executed before starting the NFS service: EXECSTARTPRE=/USR/LIBEXEC/NFS-UTILS/SCRIPTS/NFS-LOCK.PRECONFIG//start NFS Service specific command syntax: execstart= /SBIN/RPC.STATD $STATDARG # Make sure LOCKD ' s ports is resetexecstoppost=-/sbin/sysctl-w fs.nfs.nlm_tcpport=0 Fs.nfs.nlm_udpport=0[Install]Wantedby=nfs.target

As mentioned earlier, in the Red Hat RHEL7 system, systemd uses " target" instead of the " runlevel " concept.

Sysvinit Run Level	SYSTEMD Target Name	Role
0	Runlevel0.target, Poweroff.target	Shutdown
1	Runlevel1.target, Rescue.target	Single-User mode
2	Runlevel2.target, Multi-user.target	Equivalent to Level 3
3	Runlevel3.target, Multi-user.target	Multi-User Text interface
4	Runlevel4.target, Multi-user.target	Equivalent to Level 3
5	Runlevel5.target, Graphical.target	Multi-User graphical interface
6	Runlevel6.target, Reboot.target	Restart
Emergency	Emergency.target	Emergency shell

Modify the default RunLevel to "multi-user, no graphics mode":

[root@linuxprobe ~]# ln-sf/lib/systemd/system/multi-user.target/etc/systemd/system/ Default.target

Modify the default run level to "graphical mode":

[root@linuxprobe ~]# Ln-sf/lib/systemd/system/graphical.target/etc/systemd/system/default.target

It may be frustrating for readers who have studied Red Hat RHEL6 systems or have been accustomed to using commands such as service and chkconfig to manage system services, because the command to manage services in Red Hat RHEL7 system is "systemctl", But using the same method, let's make a comparison.

Systemctl commands to manage the start, restart, stop, reload, and view status of a service:

Sysvinit command (Red Hat RHEL6 system)	Systemctl command (Red Hat RHEL7 system)	Role
Service Foo Start	Systemctl Start Foo.service	Start the service
Service Foo Restart	Systemctl Restart Foo.service	Restart Service
Service Foo Stop	Systemctl Stop Foo.service	Stop Service
Service Foo Reload	Systemctl Reload Foo.service	Reload configuration file (does not terminate service)
Service Foo Status	Systemctl Status Foo.service	View service Status

Systemctl set the command to start, not start, and view the service startup status at all levels:

Sysvinit command (Red Hat RHEL6 system)	Systemctl command (Red Hat RHEL7 system)	Role
Chkconfig foo on	Systemctl Enable Foo.service	Boot auto Start
Chkconfig foo off	Systemctl Disable Foo.service	Boot does not start automatically
Chkconfig Foo	Systemctl is-enabled Foo.service	To see if a particular service is powered-up
Chkconfig--list	Systemctl List-unit-files--type=service	viewing startup and disabling of services under various levels

SYSTEMD initialization process (RPM)