Workaround using Variable PATH Declarations in systemd.path Unit Files
Systemd is an open-source system and service manager for Linux systems. It provides various unit files that help system administrators manage and control different system services and tasks. One such unit file is the systemd.path file, which supervises and controls system paths and directories. This article will explore a workaround using variable PATH declarations in systemd.path unit files.
Understanding systemd.path Unit Files
The systemd.path file type is designed to manage and supervise a system path, directory, or file by monitoring it for changes or events using the Linux inotify mechanism. When a file under observation matches the specified criteria, systemd automatically executes the assigned service unit.
[Unit]
Description=Monitor a specific path/directory
[Path]
PathModified=/path/to/monitor
PathChanged=/path/to/monitor
[Install]
WantedBy=multi-user.target
The Need for a Workaround
While the systemd.path unit files are designed to handle the path monitoring efficiently, they have some limitations, specifically with the number of paths that can be monitored. As per the systemd documentation, each monitored path consumes a certain amount of the kernel's inotify watches. Consequently, setting up many monitored paths can lead to running out of available inotify watches. In practice, running out of inotify watches can manifest as a failure of the systemd-path process.
The Workaround: Variable PATH Declarations
To overcome such limitations, a workaround can be implemented using a technique known as variable PATH declarations. By creating symbolic links to a single monitored directory, the need for multiple monitored paths is significantly reduced. With proper systemd unit file configuration, the systemd-path process can supervise a large number of paths with minimal resource utilization.
[Unit]
Description=Monitor a variable number of paths via symbolic links
[Path]
PathModified=/monitored/directory
PathChanged=/monitored/directory
[Install]
WantedBy=multi-user.target
Now, any number of directories can be monitored by creating symbolic links under the single monitored directory:
ln -s /actual/path1 /monitored/directory/
ln -s /actual/path2 /monitored/directory/
ln -s /actual/path3 /monitored/directory/
...
References
- Lennart Poettering. The systemd Book, Packt Publishing: 2015.
- Lennart Poettering. "Improving Linux' system services: patient zero"
- Thibaut Koechlin. "Using symbolic links to monitor multiple directories for events with systemd"