What is systemd?
In modern Linux systems, systemd is the most commonly used system and service manager. Its primary role is to initialize and manage system processes, services, hardware, network, and other system resources during system startup. It also provides service dependency management, logging, and other functionalities. Additionally, systemd offers several tools that make it easier for users to manage system services and resources.
How to identify systemd in a Linux system
In a Linux system, the presence of systemd can be confirmed by checking the process with PID 1. The init process, which is responsible for initializing the system and starting other processes, typically appears as process 1.
To check:
g@GitExp:~$ ps aux | grep initroot 1 0.0 0.1 225472 9344 ? Ss Dec 23 0:07 /sbin/init splash
In the history of Linux development, different distributions used different initialization systems. However, all these init systems run as process 1, so you can check the init process to see if it’s provided by systemd.
Check the symlink for init:
g@GitExp:~$ ll /sbin/initlrwxrwxrwx 1 root root 20 Mar 2 2023 /sbin/init -> /lib/systemd/systemd*
In modern Linux systems, systemd typically replaces the traditional init system. However, the init process may still appear as /sbin/init and be symlinked to the actual systemd binary.
You can also directly check the process:
g@GitExp:~$ ps -p 1 PID TTY TIME CMD 1 ? 00:00:07 systemd
Key Features of systemd
systemd comes with many modern features and advantages that make it the preferred choice for managing most Linux systems today:
Unit Mechanism: systemd uses unit configuration files to manage services, mount points, devices, and other system resources. This is the foundational mechanism upon which other systemd features are built.
Service Dependency Management: With the unit mechanism, services can be configured with explicit dependencies, allowing fine-grained control over the order in which services are started.
Parallel Service Startup: systemd supports parallel service startup, unlike traditional init systems that start services sequentially. This can significantly reduce system boot time.
Process Monitoring and Auto-Restart: systemd can monitor processes and automatically restart services if they crash. This enhances system stability.
Resource Control: systemd uses Linux kernel’s cgroups functionality to control the system’s resource usage, such as CPU, memory, disk I/O, etc. It can allocate resources more efficiently, preventing a process from consuming excessive resources and destabilizing the system.
Log Management: systemd includes an integrated logging system called journald, which collects logs from both the system and services. The journalctl command allows easy viewing and management of logs, with support for filtering and searching.
Unified Management Interface: systemd provides a unified command-line tool called systemctl for managing system services, starting, stopping, and restarting them.
Target Mechanism: systemd uses targets to manage system states. For example, multi-user.target represents multi-user mode, which is similar to traditional runlevel 3. The target mechanism gives administrators flexible control over the system’s operational state.
History of systemd
After discussing the features of systemd, we can better understand why it has become the dominant system initialization tool among various Linux init systems.
Other commonly used initialization systems include SysV Init and Upstart.
SysV Init: SysV Init first appeared in Unix systems and later became the standard init system for early Linux distributions. However, it was gradually phased out due to its slow serial startup, reliance on complex scripts, and lack of dependency management.
Upstart: Upstart, developed to improve upon SysV Init, introduced an event-driven model and supported concurrent service startups. It quickly replaced SysV Init but was eventually phased out because it failed to compete with systemd and lacked ongoing maintenance.
systemd: systemd emerged as the preferred init system due to its support for parallel startup, dependency management, integrated logging, and resource control. It also provides a unified management tool, making it the dominant system manager in modern Linux distributions.
Disadvantages of systemd
Despite its advantages, systemd has a few drawbacks:
Learning Curve: Due to its complexity and wide range of functionalities, users may need more time to learn how to configure and manage systemd services effectively.
Overly Complex Design: Some users argue that systemd’s design is too large and complex, especially its "monolithic" approach. This may lead to unnecessary features and increased system complexity.
Imperfect Compatibility with Legacy Systems: Although most modern Linux distributions now use systemd and it has worked to improve compatibility, the transition from older init systems or the porting of legacy services to systemd may still present compatibility challenges.
In conclusion, systemd stands out not only because of its superior initialization mechanism but also because it offers features and tools not found in other init systems. Over time, the advantages of systemd will continue to make it more widely used, though for users, understanding its complexity and potential drawbacks remains a challenge.