This tutorial explains the differences between SysVinit, Upstart and Systemd. Learn what the systemd is and how it is different from its predecessors.
What is the systemd?
The systemd (system daemon) is a service and management mechanism. In RHEL7 or above, it is responsible for initializing and activating system resources and server daemons at the boot time as well as on a running system.
In easy language, the systemd starts and manages all types of processes and services in Linux. In technical language, it is an initializer program that brings the system up to a defined state by activating all services and processes those are enabled in that state. To know which services and processes are enabled in which state, it uses configuration files.
SysVinit v/s Upstart v/s Systemd
Systemd is the replacement of the SysVinit and Upstart initializer programs. In RHEL6, the Upstart initialization program was used. Before RHEL6, the SysVinit was the default initialization program.
In order to understand the differences between SysVinit, Upstart, and Systemd, we have to understand the initialization process.
The kernel starts the initialization process immediately after the booting process. Since the Linux kernel uses process ID 0 to refer to itself, it assigns the process ID 1 to the initialization process.
Once started, the initialization process takes control of bringing the system up. It reads the configuration file and based on the configured state in the configuration file it starts all other services and processes. In easy language, you can say that the initialization process is the mother of all processes.
To refer to the initialization process, the SysVinit and Upstart use the term init while the systemd uses the term systemd. SysVinit, Upstart, and Systemd handle the initialization process differently.
SysVinit
This initialization process was created for 'UNIX System V' systems in the early 1980s. After a little bit of customization, it was adopted in Linux. This classic initialization program worked well for many years. Till RHEL5, RedHat used this program as the default initialization program.
This process defines six system states known as run-levels and maps all processes and services with these run-levels. It also offers easy to use commands and methods for managing run-levels and their related services.
This program was created to work within a static environment. It does not work well with hot-plug devices. For example, suppose you have a Linux system that is using this program for services and processes management. If this system is in running state and you attach a USB device to the system, this program does not recognize that USB device.
This program starts all services in a pre-defined sequence. It executes the next script in the sequence only if the current script in the sequence is executed or timed out. If a script stuck during the execution, it had to wait until that script timed out. This unexpected wait makes the entire system initialization process less efficient and ultimately slower.
Upstart
The Upstart was created as a replacement for the SysVinit model. Unlike SysVinit which had created to operate in a static environment, the Upstart was created to operate in a flexible environment.
The Upstart provides three major benefits over the SysVinit. These benefits are; event-based service management, asynchronous services startup, and automatic restart of crashed services.
The Upstart, instead of using run-levels, uses system events to start and stop services. An event is a change in the system state. When an event occurs, the upstart detects that event and makes necessary changes.
An event can be anything that requires or triggers a change in the system state. The following are a few examples of events.
- A hot-plug device (such as USB drive) is plugged-in or removed from the system.
- The system boots up.
- A service is started or stopped.
The Upstart was developed as a replacement of SysVinit, not as a modern system initializer program. Upstart fully addressed all of SysVinit's shortcomings, but missed out on new features. Until a modern initializer program has been developed, to overcome the limitations of SysVinit, the Upstart was used as a temporary solution by many Linux developers. RedHat also used the Upstart in RHEL6.
Systemd
The Systemd is the modern system initializer program. Most Linux developers including RedHat (from version 7) and Ubuntu (from version 15.04) use systemd as the default system initializer program.
Unlike its predecessors, the systemd is not easy to understand. To meet with modern system requirements, it contains several features and tools. These features and tools allow you to manage services, devices, sockets, mount points, swap area, unit types, targets, and many mores.
Advantages of the Systemd are the following.
- It starts services in parallel mode.
- It automatically resolves dependencies.
- It can repawn processes.
- It can track and relate processes together by using Linux control groups.
- It has been created to operate in the flexible modern environment. It means, it supports all types of modern hardware such as hot-plug devices and large data storages.
- It can start daemons on-demand without restarting the running service.
- It can create span-shot of the running system state.
- It can log events.
To provide above listed functions and many others, the systemd uses complex mechanism. In the next parts of this article, I will explain how the systemd works and how can you manage system through the systemd effectively.
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