Differences between SysVinit, Upstart and Systemd

The Systemd (system daemon) is a service and management mechanism. It is responsible for initializing and activating system resources and server daemons at boot time and on a running system in RHEL7 or above. Simply put, the Systemd starts and manages all types of processes and services in Linux. Technically, it is an initializer program that enables the system to a defined state by activating all services and processes. It uses configuration files to know which services and processes are enabled in which state.

SysVinit v/s Upstart v/s Systemd

Systemd is the replacement of the SysVinit and Upstart initializer programs. RHEL6 used the Upstart initialization program. Before RHEL6, the SysVinit was the default initialization program.

SysVinit, Upstart, and Systemd control the initialization process. The kernel starts the initialization process immediately after the booting process. 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 other words, 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. It 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 related services.

It 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 using this program for services and process management. If this system is running and you attach a USB device to the system, this program does not recognize that USB device. It starts all services in a pre-defined sequence. It executes the following script in the sequence only if the current script 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 was designed to operate in a static environment, Upstart was developed to operate in a flexible environment. It provides three significant benefits over the SysVinit. These benefits are event-based service management, asynchronous services startup, and automatic restart of crashed services.

Instead of using run-levels, it 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 any incident that requires or triggers a change in the system state. The following are a few examples of events.

• A hot plug device (such as a USB drive) is plugged in or removed from the system.
• The system boots up.
• A service is started or stopped.

The Upstart was developed to replace SysVinit, not as a modern system initializer program. Upstart addressed all of SysVinit's shortcomings but missed out on new features. Upstart was used as a temporary solution until a modern initializer program was developed to overcome the limitations of SysVinit. RedHat used the Upstart in RHEL6.

Systemd

The Systemd is the modern system initializer program. 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 straightforward. To meet modern system requirements, it contains several features and tools. These features and tools allow us to manage services, devices, sockets, mount points, swap areas, unit types, targets, etc.

The advantages of Systemd

• 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 a flexible modern environment. It supports all types of modern hardware, such as hot-plug devices and ample data storage.
• It can start daemons on-demand without restarting the running service.
• It can create a span shot of the running system state.
• It can log events.

To provide the above-listed functions and many others, the Systemd uses complex mechanisms. The following parts of this tutorial explain it.

Conclusion

SysVinit was the first initialization program on Linux. Upstart replaced it to meet modern requirements. Systemd is the updated version of Upstart. SysVinit served its purpose in a simpler, static environment with a straightforward approach to service management. However, it struggled with dynamic hardware and service dependencies. Upstart was developed as a bridge between SysVinit and Systemd. It supports modern hardware and starts services parallelly. However, it does not fulfil all modern requirements. Systemd replaced Upstart. It provides all the features and functions modern systems require. It offers parallel service startup, dependency resolution, logging mechanism, process and service management, and many other features.