Init vs systemd
Definition of systemd
- According to Lennart Poettering, the developper of systemd:
- "systemd is a system and session manager for Linux, compatible with SysV and LSB init scripts. systemd provides aggressive parallelization capabilities, uses socket and D-Bus activation for starting devices."
- In "Simpler Language":
- systemd is a more efficient method of controlling processes. It has the flexibility to start services in parallel, and have them communicate with each other, even if they are restarted (respawned). This is particularly useful for system administrators to temporarily service or update services without affecting the other dependent services.
The Old Way: init
Why Switch to systemd?
- Why switch to systemd when the concept of init and run levels seemed to work for 40 years?!? If it isn't broke, why try to fix it?!?
- Yes, the concept is very easy to understand. On the other hand, with modern operating systems, there is pressure to "evolve" into more efficent running operating systems. For example, in many ways both Apple creators and Unix/Linux creators have influenced each other to build better operating systems over the past decade. The Apple Mac OSX operating system uses a variation of the Unix kernel. On the other hand, Unix/Linux developpers have noticed Apple's method of running services in parallel as opposed to in sequence.
- Here is an example comparing init vs systemd:
- (Reference: (beyond init: systemd) )
- The init process may force certain services to be launched in a particular sequence. For example, the Bluetooth process cannot launch until the avahi process launches. On the other hand, the avahi process cannot launch until the D-BUS process launches, which cannot start until the syslog process is launched.
| syslog V | D-Bus V | Avahi V | BlueTooth V
- The traditional method can take time, and cause problems if a dependent process restarts.
- On the other hand, systemd launches all process in parallel (at same time), but allows them to communicate with each other (via sockets) for "on-demand" communication. This helps to improve efficiencies when running services, and makes it easier for system administrators to update services without "side effects" to the other dependent services.
| syslog | D-Bus | Avahi | BlueTooth V V V V
- Note: Although Linux operating systems (other than Fedora) still use init, systemd is compatible with init, but systemd is likely to replace init for all Unix/Linux distributions in the future...
The New Way: systemd
- Services are organized in terms of units which consist of a name and a type which correspond to a configuration file.
- There are seven different types of units:
|Processes (daemons) start/stop/restart/reload|
|socket||Define communication channel (for other services)|
|device||Recognise hardware device (plug & play)|
|mount||Connect device to file system|
|automount||Automatically connect device to file system (bootup)|
|target||Groups related services (dependency)|
|snapshot||Reference other units|
- NOTE: For this tutorial resource, we only focus on the service unit.
systemd Command Usage
- Below are examples of systemctl commands:
|Listing Running Services||systemctl list-units|
|Display Status of all Services
(pipe to grep for just one service)
|systemctl list-units --all|
|Confirm Status of Running Service||systemctl status name.service|
|Stop Service||systemctl stop service-name|
|Start Service||systemctl start service-name|
|Restart Service||systemctl restart service-name|
|Display Service Status||systemctl status service-name|
|Enable Service||systemctl enable service-name|
|Disable Service||systemctl disable service-name|
Changing the Runlevel
According to WIKIPedia (http://en.wikipedia.org/wiki/Runlevel)"
- "Runlevel" defines the state of the machine after boot. Different runlevels are typically assigned to:
- single-user mode
- multi-user mode without network services started
- multi-user mode with network services started
- system shutdown
- system reboot
- The exact setup of these configurations will vary from OS to OS, and from one Linux distribution to another.
Refer to the following link to change the runlevel of your system using systemctl:
- YouTube Video: beyond init: systemd